Several studies have demonstrated effects of stimulus-stimulus pairing (SSP) on children's vocalizations, but numerous treatment failures have also been reported. The present study attempted to isolate procedural variables related to failures of SSP to condition speech sounds as reinforcers. Three boys diagnosed with autism-spectrum disorders participated. Phase 1 was designed to assess SSP effects on production of auditory stimuli via button pressing. When SSP failed to produce a preference for the target stimulus, we instituted a series of procedural manipulations intended to address potential reasons for failure. One participant preferred the target stimulus when given the opportunity to select preferred items for pairing prior to each session, but a subsequent reversal attempt produced ambiguous results. Two participants showed no consistent preference in Phase 1 and underwent a within-session reinforcer evaluation in Phase 2, in which alternative controlling variables were demonstrated by delivering preferred stimuli contingent on button pressing. (Contains 2 tables and 3 figures.)