This study investigated the effects of introducing the Picture Exchange Communication System (PECS) on the frequency of requesting and commenting and the length of verbal utterances of two children with autism spectrum disorder (ASD) who presented with some spoken language, but limited use of language in communicative exchanges. A mixed research design was used, including a quantitative component--a single-subject multiple-baseline design (MBD) across three behaviours, repeated with two participants--and a qualitative component. Data was collected in the PECS pre-training, training, post-training and follow-up stages, in both structured and unstructured settings. The quantitative data was visually represented and analysed to determine the effectiveness of the PECS. The qualitative component investigated the impact of the PECS on other areas (e.g. communication profile, speech complexity and pragmatic skills), and included parent and educator perspectives. Both participants benefited from the introduction of PECS. The findings indicated highly effective treatment for requesting and mixed results for commenting and length of verbal utterances. There were considerable increases in intentional communicative acts (ICAs) for both participants, with marked increases in requesting (function) and the development of forms of communication (from augmentation of speech with pictures to speech only utterances). Clinical, educational and research implications were raised. (Contains 1 note, 3 tables, and 5 figures.)