This article describes episodes of concurrent metalinguistic and ideational thinking in the verbal reports of 23 adult ESL learners composing on two tasks, then relates these descriptions to claims about the value of composition writing for second language learning. Three kinds of thinking episodes, appearing in about 30% of the decisions reported by learners while composing, show potential value for incidental learning of the second language: (a) searching out and assessing appropriate wording, (b) comparing cross-linguistic equivalents, and, much less frequently, (c) reasoning about linguistic choices in the second language. Multivariate analyses indicated that the frequency of these thinking episodes is significantly related to learners' writing expertise in their mother tongue. Implications are drawn for refining Swain's 1985 notion of “comprehensible output” in view of other theories of cognitive learning and second language acquisition, a necessary preliminary to empirical assessment of this hypothesis.