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