This article reports two experiments which examined the utility of the phoneme monitoring technique for studying syntactic processing of sentences. In French, by using self-embedded relative clauses, it is possible to isolate and examine the effect of a syntactic cue while controlling the factors known to effect phoneme detection times. Monitoring within and after the relative clause led to significant differences in phoneme detection times for reversible subject and object relatives only after the clause boundary. These results demonstrate the sensitivity of the phoneme monitoring task to syntactic processing and are taken to reflect structural calculations of the underlying grammatical relations for the reversible object relatives. When lexical information was introduced with nonreversible relatives, there was no longer a difference between the detection times for subject and object relatives after the clause boundary. Thus, it appears that lexical information can be used in the attribution of underlying grammatical roles.