This article examines sentence matching, a methodology frequently used in the second language (L2) literature to determine notions of grammaticality of nonnative speakers (NNS). Native speakers (NS) of French and L2 learners of French performed a sentence-matching task focusing on three areas of French grammar: adverb placement, subject–verb agreement and clitic-pronoun placement. In sentence-matching tasks participants respond to two sentences on a computer screen indicating whether the two sentences are identical or not. In general, grammatical sentences are responded to faster than ungrammatical sentences and have been used in the L2 literature as a way of determining grammatical knowledge. The results from the NSs of French show that when there is a high degree of difficulty in interpretation of ungrammatical sentences, sentence matching is a useful tool for determining grammaticality. For NNSs there is little evidence that sentence matching predicts grammaticality. A traditional acceptability judgement task was administered to NNSs. Sentence-matching did not correlate with NNS’s individual notions of grammaticality. Issues of proficiency level and the nature of ungrammatical sentences are important determinants when considering the validity of sentence-matching as a research tool.