--- - |2 We evaluated whether contextual control over equivalence and nonequivalence and responding by exclusion can explain the outcomes of relational frame theory (RFT) studies on sameness and opposition relations. We trained nine college students to maintain and reverse conditional discriminations with X1 and X2 as contextual stimuli. In Experiment 1, X1 and X2 controlled derived stimulus relations (DSR) analogous to those controlled by Same and Opposite in RFT studies. These results can be explained by at least two hypotheses: X1 and X2 were cues for equivalence and nonequivalence and responding by exclusion, or for sameness and opposition. In Experiment 2, X1 and X2 controlled DSR predicted by the hypothesis that they were cues for equivalence and nonequivalence and responding by exclusion, and not predicted by the hypothesis that they were cues for sameness and opposition. The results of Experiment 2 and the functional equivalence of X1 and X2 with Same and Opposite in Experiment 1 suggest that Same and Opposite were cues for equivalence and nonequivalence and responding by exclusion in RFT studies. - Journal of the Experimental Analysis of Behavior, Volume 110, Issue 2, Page 213-228, September 2018.