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The Merck Index is a reference work on chemicals, drugs and biologicals, and it has been regarded for more than 130 years as a key source of physical, pharmacological and historical information on them. Since 2013 it has been updated by the Royal Society of Chemistry (RSC), and its online edition, The Merck Index Online, holds over 12,000 monographs. Researchers use it to confirm what a substance is called, what its basic properties are and where to find the key literature. It is a different thing from the pharmaceutical companies that share the Merck name: according to the RSC, the name THE MERCK INDEX is owned by Merck & Co., Inc., Rahway, NJ, USA and its affiliates, and is licensed to the RSC for use in the USA and Canada. For the wider subject, see the guide to what chemistry is.
What Is The Merck Index?
The Merck Index is an encyclopedia of chemicals, drugs and biologicals. Each entry is a monograph on a single substance or a group of related compounds. A first edition appeared in 1889, issued by the German company E. Merck, and the work has been republished in many print editions since. Today it exists as print editions and as the subscription-based online edition at merckindex.rsc.org, which the RSC describes as continuously expanded with expert-curated content that highlights only the most relevant literature and patents.
A monograph typically records the substance’s CAS Registry Number, synonyms and systematic names, formula and molecular weight, structural formula, appearance, melting and boiling points, solubility, citations to the synthesis literature, a therapeutic category where one applies, and caution and hazard information. This is general reference content about substances, and it is not a source of advice on using any product.
Who Runs It and What It Covers
The Royal Society of Chemistry, the UK learned society for chemistry, has updated The Merck Index since 2013. Editorial enquiries go to [email protected] and sales and access enquiries to [email protected], as listed on the site. The online edition covers chemicals, drugs and biologicals and states a size of over 12,000 monographs. Its homepage lists these sections: Basic search, Advanced search, Browse monographs and Reference tables.
When you write up property data you have taken from a reference work, follow the conventions of your target journal; the guide to ACS-style chemistry manuscript conventions covers one widely used set.
How to Search or Use It
The Merck Index Online is a subscription resource, so access normally comes through an institution or a licence arranged with the RSC. The site has a Sign in link at the top of every page. The interface terms below are the ones shown on the public homepage.
- Basic search: enter a name, a CAS Registry Number or a molecular formula in the search box on the homepage.
- Advanced search: use this section when a single name or number is not enough to narrow the results.
- Browse monographs: go through the collection of monographs instead of searching for a specific substance.
- Reference tables: open these for the supporting tables the site provides alongside the monographs.
- Check the update date: the homepage shows a “Last updated” date, which tells you how current the content is when you cite it.
Variants of the query such as “Merck Index online” refer to this same service at merckindex.rsc.org.
Strengths and Limits
- Strengths:
- Entries are expert-curated, and the RSC says the content highlights the most relevant literature and patents.
- One monograph gathers identifiers, physical properties and literature pointers for a substance in a single place.
- The online edition is continuously expanded, so it can include material that a print edition cannot.
- Limits:
- Access is by subscription or arranged licence, so it is not open to everyone.
- It is a selective reference with over 12,000 monographs, not an exhaustive registry of every known compound.
- Print editions are fixed in time, so for current content use the online edition.
Alternatives
Other chemical resources cover different needs. Our guides to each go into detail on what they hold and how to search them.
| Platform | Maintainer | Access model | Use it for |
|---|---|---|---|
| The Merck Index Online | Royal Society of Chemistry | Subscription | Curated monographs on chemicals, drugs and biologicals |
| PubChem | NCBI / NIH | Free to use | Broad compound exploration and bioassay records (PubChem guide) |
| SciFinder | Chemical Abstracts Service | Subscription | Chemical literature and substance searching (SciFinder guide) |
| Reaxys | Elsevier | Subscription | Reaction and property data (Reaxys guide) |
Frequently Asked Questions
What is The Merck Index?
It is an encyclopedia of chemicals, drugs and biologicals made up of monographs on individual substances. The RSC describes it as having been regarded for over 130 years as a source of key physical, pharmacological and historical information on these substances.
What is the latest edition of The Merck Index?
The latest numbered print edition is the 15th, published in 2013 and the first under the Royal Society of Chemistry. The online edition is updated continuously, and its homepage shows a “Last updated” date; the date shown at the time of checking was 4 November 2026.
How to reference The Merck Index?
Follow the citation style your journal, publisher or institution requires, and take the details from the edition you used. For the print work, record the title, edition number, editors, publisher, year and the entry or monograph you used. For the online edition, record the database name, the publisher (Royal Society of Chemistry), the URL and the date you consulted it. A cited property value is only as current as the edition it came from, so name the edition.
What does Merck stand for?
Merck is not an acronym. It is the name of the Merck family company in Germany that issued the first edition in 1889. Today the name THE MERCK INDEX is owned by Merck & Co., Inc., Rahway, NJ, USA and its affiliates and licensed to the RSC for use in the USA and Canada.
Sources
- Royal Society of Chemistry. The Merck Index Online. https://merckindex.rsc.org/ (checked 2026-11-07).
- Wikipedia. Merck Index (editions and history). https://en.wikipedia.org/wiki/Merck_Index (checked 2026-11-07).
