This article studies the relationship between synonymy and equivalence. The author considers the linguistic behaviour of two sets of potential synonyms in English and German texts from the domain of genetic engineering, based on a corpus of texts aimed at a scientific but not necessarily expert readership. The analysis suggests that the notion of synonymy, and by extension that of equivalence, needs to take into account a number of factors at the linguistic level which are not usually considered in terminology work or indeed in lexicographical work. These factors include the role of the grammatical category number and the role of the combining elements in compound formation as head or modifier.