We examined the effects of peer presence on the conditioning of new reinforcers via observation. At the outset, strings and toothpicks did not reinforce maintenance or acquisition responses for 3 preschoolers with and without developmental delays. In the first intervention, the stimuli were placed in a cup in front of an empty chair next to the participant, and the participant was denied access to those stimuli. The second intervention was identical to the first, except that a peer was present. Postintervention tests revealed that only the peer intervention was successful in conditioning neutral stimuli as reinforcers for both maintenance and acquisition responses for all 3 participants. The presence of a peer appears to facilitate the conditioning of reinforcers by observation.