This study compares use of the Picture Exchange Communication System (PECS) and a Voice Output Communication Aide (VOCA) with four preschool children who were either non-speaking or limited in their ability to speak and did not use an AAC system to communicate functionally. An alternating treatment single subject design was used to measure participants' preferences for each system and the verbalizations of the participants during system use. Results indicated that participants learned PECS in a relatively short time period, preferences for one mode of communication are not predictable, and the influence of the communication systems on each participant's verbalizations varied. (Contains 4 figures and 8 tables.)