Previous studies comparing the effectiveness of the stimulus-pairing-observation and matching-to-sample procedures in facilitating the formation of equivalence relations have had conflicting findings. In an attempt to clarify the reasons for this, Experiment 1 replicated one of the experiments from Leader and Barnes-Holmes (2001b) but with the Chinese characters used by Clayton and Hayes (2004) as stimuli. The adult participants completed both the stimulus-pairing-observation and matching-to-sample procedures. Neither procedure was found to be more effective than the other, with few of the participants demonstrating equivalence after either procedure. Due to the failure of most participants to demonstrate equivalence, Experiment 2 replicated Experiment 1 but with the original nonsense syllables used by Leader and Barnes-Holmes (2001b). Equivalence was not demonstrated by any of the participants in Experiment 2. Therefore, the failures in Experiment 1 were not the result of the stimuli used. The use of the same stimuli in conflicting relations was identified as the most likely cause. Experiment 3 addressed this by using different nonsense syllables with each procedure. This resulted in greater accuracy on both the symmetry and equivalence tests compared to the earlier experiments; however, none of the participants demonstrated equivalence, and the procedures did not differ in their effectiveness. In Experiments 4 to 6 participants experienced either the stimulus-pairing observation or matching-to-sample procedures. These three experiments examined the effect of instructional specificity, stimulus arrangement, and the number of training trials on the effectiveness of these two procedures. Experiment 4 found that instructions which outlined the task required more specifically increase the effectiveness of both procedures marginally, and that a larger number of training and testing cycles (compared to e 1-3) did not aid in the development of equivalence. Experiment 5 examined the effectiveness of the many-to-one or one-to-many stimulus arrangements (compared to the linear arrangement used in the earlier experiments). The many-to-one and one-to-many arrangements resulted in more participants demonstrating equivalence than the linear arrangement for both the stimulus-pairing-observation and matching-to-sample procedures. Experiment 6 replicated E 5 but with more training trials prior to each equivalence test. This resulted in more participants demonstrating equivalence across both procedures and all stimulus arrangements. The stimulus-pairing-observation and matching-to-sample procedures were found to be equally effective in terms of accuracy achieved on the equivalence tests; however, the matching-to-sample procedure resulted in the development of equivalence within fewer training trials than the stimulus-pairing-observation procedure. When the stimulus-pairing-observation procedure was used, more participants demonstrated equivalence with the one-to-many arrangement than with the many-to-one or linear arrangements. When the matching-to-sample procedure was used, the one-to-many and many-to-one arrangements resulted in more participants demonstrating equivalence than the linear arrangement. Comparisons across the experiments suggested that the number of training trials completed prior to each equivalence test, but not the total number of training trials completed, affected performance. The effectiveness of the stimulus arrangements differed across the procedures, but one-to-many arrangement was more effective than the linear arrangement for both procedures. Overall, these experiments suggest that there is little difference in the effectiveness of the MTS and SPO procedures in facilitating the formation of equivalence relations, and that the development of equivalence is made more likely for both procedures by the addition of more training trials prior to each test, and the use of a one-to-many stimulus arrangement.