Synonymic variation is a common resource in specialized languages, particularly in legal language (Acuyo Verdejo 2004; Thiry 2009a; Barceló Martínez 2010). Since it is a semantic strategy, an erroneous interpretation can change the meaning of the text and the legal relationships created therein. Therefore, it is essential to detect it and manage it adequately. This paper takes a closer look at the use of synonymic variation within the legal field to understand how it works in English and Spanish for the purpose of setting up similarities and differences between them. Our research will be carried out through the analysis of comparable corpora of sales agreements in both languages. A series of textual markers will be used to detect the cases of synonymic variation in the texts in order to set a list of patterns of use regarding frequency, productivity, functioning and equivalence relationships created between synonyms. The results will help us understand how synonymic variation could affect the understanding and interpretation of legal texts, specifically regarding translation, where the management of this strategy can be essential to accurately transfer the meanings between source and target text.