The present study is a follow-up to a classroom experiment with university ESL students that demonstrated incidental acquisition of new lexical knowledge through the reading of thematically related texts. Introspective data from similar students using the same materials are analyzed in this study to explore how vocabulary knowledge may be acquired as a by-product of reading for comprehension. The researchers sought to identify the strategies and the kinds of knowledge and information 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 in inferring word meanings from various textual and other cues was developed, which provided a framework for describing learners' inferencing behavior. Findings are interpreted in terms of existing research and theory on incidental vocabulary acquisition within an input-processing framework. Pedagogical implications are drawn.