Current theories of morphosyntax propose that features play a key role in determining some aspects of word order and in the argument structure of verbs. Thus, knowledge of language consists of features and mechanisms that ensure that elements in a clause are assembled correctly for interpretation and phonological processing. Therefore, the nature of the grammar of an L2 can be explored by assessing whether features and the mechanisms through which they are assembled and checked is ‘impaired’ or not. This article assumes that processing performance can provide a window onto this grammatical competence. Evidence is presented from online processing of English by thirty Chinese-speaking, twenty-eight Japanese-speaking, and forty-six Spanish-speaking participants which shows that the basic mechanisms of grammar remain intact for L2 learners in spite of inferior performance on judgements of those same sentences. Data suggest that working memory, as measured by the reading-span test (Daneman and Carpenter 1980), is not a source of individual variation in online L2 performance, whereas word span might be.