Fifty participants were exposed to a simple discrimination-training procedure during which six S1 functions were established for six arbitrary stimuli, and S2 functions were established for a further six stimuli. Following this training, each participant was exposed to one of five conditions. In the S1 condition, participants were exposed to a stimulus equivalence training and testing procedure using only the six S1 stimuli as samples and comparisons. In the S1/S2 condition, participants were exposed to the same training and testing sequence as in the S1 condition, the difference being that three S1 and three S2 stimuli were used as sample and comparison stimuli, with each set of three corresponding to the trained equivalence relations. In the S1/S2 mixed condition, the S1 and S2 stimuli were assigned to their roles as samples and comparisons in a quasi-random order. In the S2 condition, all six S2 stimuli were used. The no-function condition served as a control condition and employed stimuli for which no stimulus-control functions had been established. The results showed that, on average, participants required more testing trials to form equivalence relations when the stimuli involved were functionally similar rather than functionally different. Moreover, participants required more test trials to form equivalence relations when novel arbitrary stimuli, rather than functionally distinct stimuli, were used as samples and comparisons. The speed of acquisition of stimulus equivalence was also related to the number of functionally similar stimuli established before training. These findings indicate a variety of ways in which the emergence of equivalence relations is affected by the functional classes in which the relevant stimuli participate.