This study investigates learners' knowledge of word classes (i.e. noun, verb, adjective) in their second language (L2). Although some L2 studies have examined the problem of word class indirectly through a focus on vocabulary and the teaching of derivational morphology (Morin, 2003, 2006; Schmitt & Zimmerman, 2002), little is known about learners' sensitivity to word class contrasts (e.g. "fuerte" "strong" versus "fuerza" "strength"). The present study utilised concurrent and retrospective think-aloud protocols to examine learners' attention to different sorts of cues to word class in Spanish. The participants, beginning-level learners of Spanish (n = 35), completed a forced-choice task that targeted word class contrasts in various syntactic contexts. The results indicate that learners attended to grammatical cues frequently, with these accounting for 45% of the total cue usage. Among the various grammatical cues, learners showed a marked preference for inflections (i.e. gender and number). The use of semantic cues was also common (32%), but led to word class confusions when learners knew only the general meaning of a target word but not the exact L1 equivalent. Finally, an error analysis indicates that the main problem for these learners may not be misclassification of lexical items but rather incomplete syntactic knowledge. (Contains 4 tables and 5 notes.)