ISO 18626 (Interlibrary Loan Transactions — Application Protocol)

ISO 18626 (Information and documentation — Interlibrary loan transactions) is the ISO application protocol that lets library systems automatically exchange interlibrary loan (ILL) and resource-sharing messages with each other, instead of staff re-keying requests by phone, fax, or email. It defines the message set, the data elements each message carries, and the permitted sequences and states a transaction can move through, so that a request created in one library’s system can be transmitted to, and acted on by, a completely different vendor’s system at the lending library.

What makes a transaction ISO 18626-compliant

An interlibrary loan transaction is operating under ISO 18626 when the request and its subsequent status updates are carried as structured, machine-readable XML messages conforming to the standard’s schema, exchanged over a web-services transport — not as unstructured correspondence. This is the key operational distinction from the pre-web ILL protocols it succeeds: ISO 10160 and the associated ISO 10161-1/10161-2 messaging standard, which were built for an OSI-era, non-web messaging environment and saw comparatively limited real-world implementation. ISO 18626 was designed specifically to be practical to implement in a modern, XML- and web-services-based technology stack, which is a large part of why it has seen substantially broader adoption among integrated library systems and ILL management platforms than its predecessor did.

The core message types

ISO 18626 defines a small set of message types that cover the lifecycle of a loan or copy request:

• Request — sent by the requesting library’s system to ask a supplying library to lend an item or supply a copy.
• Supplying Agency Message — sent by the supplying library’s system to communicate status (for example, that the item has shipped, is unavailable, or has been renewed) and to take actions such as cancelling or conditioning the request.
• Requesting Agency Message — sent back by the requesting library’s system, for example to confirm receipt, request a renewal, or report the item as returned or lost.

Confirmation messages acknowledge receipt of each of these at the transport level. Together, this small, fixed vocabulary of message types is what allows two independently developed library systems — built by different vendors, running different software — to interoperate on a single transaction without a custom point-to-point integration.

Examples

A library’s resource-sharing system sending a Request message, formatted per the ISO 18626 XML schema, to a lending library’s system over a web-services connection — with the lending system replying via a Supplying Agency Message reporting the item shipped — is an ISO 18626 transaction. A shared, ISO 18626-based registry of standardized codes (for example, for loan conditions or unavailability reasons) used consistently across implementing systems so that a status reported by one system is interpreted the same way by another is also part of how the standard is put into practice; the standard’s own implementer community maintains such a registry alongside the schemas and conformance guidance for adopting systems.

Counter-example

A library emailing a PDF request form to another library’s ILL office, or a staff member manually re-keying a request into a second system’s web form, is not an ISO 18626 transaction — no structured, standard-conformant message is exchanged between systems, even though the underlying business activity (borrowing an item from another library) is the same. Likewise, a proprietary, vendor-specific ILL API that is not built against the ISO 18626 schema and message set does not make a transaction ISO 18626-compliant, even if it is broadly similar in function.

Why it matters for resource sharing

Before ISO 18626, interoperability between different ILL and resource-sharing platforms depended heavily on either shared vendor infrastructure or manual staff effort, which limited how easily libraries on different systems could lend to and borrow from each other. A common, web-native application protocol lowers that integration cost: any system that implements ISO 18626 correctly can, in principle, transact with any other conformant system, which is what has made it a practical foundation for cross-network and cross-vendor resource sharing rather than a standard that only works within a single library consortium’s shared platform.

Related standards

ISO 18626 sits alongside other library- and publishing-standards work that reduces transaction friction between institutions and vendors, such as SERU (Shared E-Resource Understanding), the NISO recommended practice that removes the need for a bespoke license agreement on lower-value e-resource subscriptions. Both are examples of NISO Recommended Practice-style and ISO standards work aimed at making routine library transactions — borrowing an item versus subscribing to a resource — less costly to execute at scale.