Treaty amendments constitute a critical but under-researched aspect of international law. In this article, we present a comprehensive survey of 491 amendment procedures across 691 multilateral environmental agreements. We use this data collection to build a typology of amendment procedures based on various combinations of control, adaptability, and flexibility. We introduce the property space reduction method as a valuable tool for building typology and analysing international law. We find a clear trend towards the inclusion of amendment procedures, which makes treaties increasingly adaptable. This adaptability is generally coupled with flexibility to avoid infringing on consent. As a result, amended treaties risk being increasingly fragmented into differentiated bundles of obligations split among subsets of members. We also examine how key features of treaty membership, such as power distribution, correlate with the occurrence and types of amendment procedures.
