Word frequency and semantic priming effects are among the most robust effects in visual word recognition, and it has been generally assumed that these two variables produce interactive effects in lexical decision performance, with larger priming effects for low-frequency targets. The results from four lexical decision experiments indicate that the joint effects of semantic priming and word frequency are critically dependent upon differences in the vocabulary knowledge of the participants. Specifically, across two Universities, additive effects of the two variables were observed in means, and in RT distributional analyses, in participants with more vocabulary knowledge, while interactive effects were observed in participants with less vocabulary knowledge. These results are discussed with reference to [Borowsky, R., & Besner, D. (1993)." Visual word recognition: A multistage activation model." "Journal of Experimental Psychology: Learning, Memory, and Cognition, 19," 813-840] multistage account and [Plaut, D. C., & Booth, J. R. (2000). "Individual and developmental differences in semantic priming: Empirical and computational support for a single-mechanism account of lexical processing." "Psychological Review, 107," 786-823] single-mechanism model. In general, the findings are also consistent with a flexible lexical processing system that optimizes performance based on processing fluency and task demands. (Contains 10 tables and 6 figures.)