In Experiment 1, four subjects were trained to match two visual samples (A) and their respective nonidentical visual comparisons (B); i.e., A-B matching. During nonreinforced test trials, all subjects demonstrated stimulus equivalences within the context of sample-comparison reversibility (B-A matching): When B stimuli were used as samples, appropriate responding to A comparisons occurred. A-B and B-A matching persisted given novel stimuli as alternate comparisons. However, the novel comparisons were consistently selected in the presence of nonmatching stimuli: i.e., during trials comprised of a novel comparison, an A or B sample from one stimulus class, and an "incorrect" comparison from the other, B or A stimuli respectively. In Experiment 2, three groups of subjects were trained under three different mediated transfer paradigms (e.g., A-B, C-B matching). Tests for reversibility (e.g., B0A, B0C matching) and mediated transfer (e.g., A-C, C-A matching)evinced stimulus equivalences for 11 of 12 subjects. The 11 subjects also matched the mediated equivalences given novel comparisons; whereas, they selected the novel comparisons when combined with nonmatching stimuli. Overall, the demonstrated stimulus equivalences favor a concept learning interpretation of non-identity matching-to-sample. Additionally, the trained and mediated matching relations were comprised of complementary sets of S+ and S- rules: Any stimulus of a given class used as a sample designated both the "correct" and "incorrect" comparisons.