Author co-citation analysis (ACA) has been used to explore changes in the intellectual base of the information retrieval (IR) field over two consecutive time periods: 1987-1991 and 1992-1997. Thirty-nine highly cited IR researchers were selected as the research sample. Multidimensional scaling (MDS) and clustering techniques (CT) were used to create the two-dimensional maps to display the dynamic intellectual structure of IR, based on scholars citing their work over these two time periods. Factor analysis (FA) has been used to reveal the ‘breadth’ of the authors’ research areas. ACA offers a good technique that contributes to the understanding of intellectual structure in the sciences and possibly in other areas to the extent that those areas rely on formal scholarly communication such as serial publications. Nonetheless, obvious drawbacks exist in ACA. These include the subjective nature of the interpretation of results, the difficulty of readily identifying clusters and the inability to distinguish collaborative research relationships between authors. Thus, ACA by itself is insufficient. However, ACA can be enhanced significantly when combined with FA to give a more accurate and useful picture of the MDS results.