Direct comparison
JSON vs XML for Research Data and APIs
JSON vs XML compared against RFC 8259, W3C XML 1.0 and XSD 1.1: types, comments, encoding, duplicate names and schemas, plus when to pick each.
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How do JSON, XML compare side by side?
The table below compares JSON, XML across 10 procurement-relevant dimensions, from specification through source.
Side-by-side comparison
| Dimension | JSON | XML |
|---|---|---|
| Specification | RFC 8259 defines JSON as "a lightweight, text-based, language-independent data interchange format." | W3C XML 1.0 (Fifth Edition) defines XML as "a subset of SGML that is completely described in this document." |
| Data model | Two structured types, objects and arrays, built on four primitives: strings, numbers, booleans and null (RFC 8259, section 1). | Elements and attributes in a document tree; XML 1.0 describes the document syntax, and typing comes from a schema language. |
| Built-in value types | Numbers, strings, booleans and null are part of the format. Literal names true, false and null must be lowercase (section 3); implementations may limit number range and precision, and Infinity and NaN are not permitted (section 6). | Content is text. Typed values (dates, integers and so on) come from a schema such as XSD, whose simple types constrain text values. |
| Comments | The RFC 8259 grammar defines no comment syntax. | Comments are part of XML 1.0; the string "--" must not occur inside one (section 2.5). |
| Character encoding | JSON text exchanged between systems outside a closed ecosystem must be encoded as UTF-8 (RFC 8259, section 8.1). | Every XML processor must be able to read UTF-8 and UTF-16 (XML 1.0, section 4.3.3). |
| Duplicate names | Names within an object "SHOULD be unique"; when they are not, receiving software behavior "is unpredictable" (RFC 8259, section 4). | An attribute name must not appear more than once in the same start-tag (XML 1.0, section 3.1). |
| Schema language | JSON Schema is a separate, versioned specification; its site lists 2020-12 as the current version. It is not part of the JSON format itself. | XSD 1.1 is a W3C Recommendation for "describing the structure and constraining the contents of XML documents," including those that use XML namespaces. It is separate from XML 1.0. |
| Namespaces | RFC 8259 defines objects and arrays only; it has no namespace mechanism. | Defined by a separate W3C Recommendation, Namespaces in XML, which uses colon-prefixed names. |
| When to pick each | Pick JSON when the consumer wants a small set of typed values in objects and arrays, such as responses from a web API. Pick it when a JSON Schema already describes the payload you must produce. | Pick XML when a community standard or repository already fixes XML plus an XSD as the exchange format, or when you need comments and namespaces in the document itself. Examples on CASRAI include the Define-XML component of CDISC standards and DDI Codebook. |
| Source | RFC 8259 (rfc-editor.org/rfc/rfc8259); W3C, Extensible Markup Language (XML) 1.0 Fifth Edition, 26 November 2008 (w3.org/TR/xml/); W3C XML Schema Definition Language (XSD) 1.1 Part 1, 5 April 2012 (w3.org/TR/xmlschema11-1/); JSON Schema specification page (json-schema.org/specification). Read 2026-10-29. | RFC 8259 (rfc-editor.org/rfc/rfc8259); W3C, Extensible Markup Language (XML) 1.0 Fifth Edition, 26 November 2008 (w3.org/TR/xml/); W3C XML Schema Definition Language (XSD) 1.1 Part 1, 5 April 2012 (w3.org/TR/xmlschema11-1/); JSON Schema specification page (json-schema.org/specification). Read 2026-10-29. |
Common questions
Common questions about JSON vs XML
Is JSON replacing XML?
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The specifications do not say so, and this page makes no claim about adoption trends. They are separate formats with separate schema languages (JSON Schema, XSD 1.1). Some research standards are defined on XML, such as Define-XML in CDISC and DDI Codebook, while many web APIs return JSON.
Can JSON have comments?
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The JSON grammar in RFC 8259 defines no comment syntax. XML 1.0 does define comments (section 2.5).
What validates each format?
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XML documents can be validated with XSD 1.1, a W3C Recommendation. JSON uses JSON Schema, a separate versioned specification whose current version is 2020-12.
Which encoding does each require?
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RFC 8259 requires UTF-8 for JSON exchanged between systems outside a closed ecosystem. XML 1.0 requires every processor to read UTF-8 and UTF-16.
Going deeper
