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Stimulus equivalence, defined as C-A matching, was tested in 80 adults following training in a matching-to-sample task involving arbitrary A-B and B-C matching. In Experiment 1, 50 subjects, successively assigned to one of five groups, were exposed to specific stimulus material. The stimuli for subjects in the first group were Greek letters only. In the remaining groups, pictures were incorporated as A-, B-, and/or C-stimuli. The probability of equivalence was low when the stimulus material consisted only of Greek letters. For the remaining groups, the probability of equivalence varied considerably depending upon whether the A-, B, and/or C-stimuli were pictures. The results indicate that seemingly minimal procedural variations can yield markedly differential outcomes not predicted by any existing model. The results also showed that responding gradually may become consistent with a pattern other than the predicted equivalence pattern. Experiment 2 replicated the results of Experiment 1 showing (a) differential probabilities of equivalence in individual subjects, depending upon the configuration of Greek letters and pictures during training/testing, (b) consistent patterns of responding even when the responding was not in accord with equivalence, and (c) higher reaction times to comparison stimuli initially during testing. Probabilities of equivalence increased in a second exposure to the tasks involving only Greek letters, whether or not the subjects were exposed to a task with pictures prior to the second Greek-letter task. Higher reaction times initially during testing may indicate precurrent problem solving behavior prior to the selection of a comparison stimulus. The finding of delayed emergence of consistent responding suggests that even the slightest tendency toward responding that partitions the stimuli into the experimenter-planned equivalence classes may evolve into consistent responding in accord with those classes.