This empirical study examines how English-as-a-second-language (ESL) learners use their vocabulary knowledge for inferring meanings of unknown words in reading comprehension. The data, collected through interviews with young adult ESL students in Canadian universities, indicate that: (a) semantic and morphological aspects of vocabulary knowledge play an important role in learners' comprehension processes; (b) a positive relationship exists between certain aspects of learners' vocabulary knowledge and their lexical inferencing ability; and (c) in processing the meaning of unknown words, all learners looked for cues to meaning, but learners with varying depths of vocabulary knowledge tended to focus on varying strategies.