Complex syntactic structures may be difficult to recognize when produced using augmentative and alternative communication (AAC) systems that do not contain grammatical markers. The present study investigated adult English speakers' production of Subject and Object relative clause sentences using a picture/symbol-based AAC system with speech output. Most participants avoided the potential ambiguity that resulted from the absence of grammatical markers. They followed spoken English word order when encoding Object relative clause sentences, but altered this order for Subject relative clause sentences. Most participants used constituent proximity to maintain the distinction between Subject and Object relative clause sentences. Results indicate the combined effects of underlying syntactic knowledge and pragmatic variables on the AAC constituent order patterns observed.