A study investigated the role of comprehension of meaningful language input in young adults' second language learning, focusing on: (1) what kinds of measurement instruments and procedures can be used in tracking student gains in specific aspects of target language proficiency; (2) development of a reliable self-report scale capturing different vocabulary knowledge levels; and (3) use of introspection in exploring links between comprehension of meaning and acquisition of vocabulary. Subjects were 37 university students in 2 language classes, an experimental group using a comprehension-based approach and authentic texts and a control group using an integrated 4-skills approach and emphasizing grammar instruction. A series of tests were administered: baseline global language proficiency tests, two rational deletion cloze tests, a self-report of vocabulary knowledge, two measures of grammatical knowledge, and (in the comprehension-based class) an introspective task on vocabulary acquisition. Results indicate superior gains in the comprehension-based class in text comprehension and discourse processing, despite smaller gains in grammatical knowledge, suggesting that explicit grammar instruction is not needed to advance comprehension skills. However, accuracy in writing and speaking was found to be linked to grammar instruction. It is concluded that comprehension-based instruction develops receptive proficiency, but other gains appear idiosyncratic. (MSE)