The present study is a follow-up to a classroom experiment with university ESL students thatdemonstrated incidental acquisition of new lexical knowledge through the reading of thematicallyrelated texts. Introspective data from similar students using the same materials are analyzed in thisstudy to explore how vocabulary knowledge may be acquired as a by-product of reading forcomprehension. The researchers sought to identify the strategies and the kinds of knowledge andinformation learners used when dealing with new L2 words they encountered while reading.Learners tended to ignore a large proportion of the words. For those words they attended to,inferencing was the main strategy employed. A taxonomy of the knowledge sources they used ininferring word meanings from various textual and other cues was developed, which provided aframework for describing learners' inferencing behavior. Findings are interpreted in termsof existing research and theory on incidental vocabulary acquisition within an input-processingframework. Pedagogical implications are drawn.