In 2 experiments assessing acquired equivalence, human participants were initially presented with 4 cues, 2 of which were paired with 1 outcome and 2 of which were paired with a 2nd outcome. These contingencies were then reversed across several training blocks such that, although each cue was paired equally often with each of the two outcomes across blocks, cues A and B always signaled the same outcome within blocks (as did cues C and D). In both experiments, performance on a subsequent transfer discrimination was enhanced when participants were required to generalize between stimuli that had been paired with the same outcome within each block of training. Additional tests did not yield evidence of a bias toward a specific set of cue-outcome contingencies in either experiment. Moreover, interviews conducted at the end of Experiment 2 revealed that performance on the transfer discrimination was enhanced only in participants who discovered the equivalence relationships during initial training. The results challenge simple associative, and attentional, accounts of acquired equivalence and favor the view that this effect is mediated by comparisons of the similarity of adjacent cue-outcome structures.