This article reports a study about the behavior and functions of term variation in research articles (RAs) in Geology and Psychology. The aim of the study was twofold: first, to investigate the role of intra-textual term variation as a device for the representation and transfer of specialized knowledge; second, to explore whether there are disciplinary differences and whether these differences can be chalked up to different perceptions of this phenomenon among subject field experts. Two methods of data collection were combined: corpus-based analysis of 38 RAs in Spanish; and semi-structured interviews with six experts. Results show that the incidence of term variation is higher in Psychology, although both groups manifested a positive attitude toward variation. Corpus analysis and interviews confirm that term variation is used as a cognitive device, to provide information about the concept's characteristics and relationships with other concepts; and as a communicative device to avoid repetition, to accommodate to the audience and to generalize. In sum, term variation proves to be an important device for constructing and communicating specialized knowledge for both disciplines.