It has been argued that languaging plays a crucial role in learning a second language (L2). The effects of languaging, especially oral languaging (e.g., collaborative dialogue, private speech), have been tested on the learning of L2 knowledge domains. This study explored the effects of written languaging by asking 24 Japanese learners of English to write out their own explanations in Japanese of the corrections they received on a draft they had written. The effects of the type (e.g., grammar based vs. lexis based) of written languaging were then assessed by inspecting the success of immediate subsequent text revisions. Two major findings emerged. First, written languaging about direct feedback on linguistic errors in the first essay helped learners successfully correct these errors during immediate revision. Second, both lexis- and grammar-based written languaging were associated with improved accuracy. These findings support arguments that providing learners with the opportunity to language about or reflect on their developing linguistic knowledge in the course of L2 learning mediates L2 learning and development. Theoretical and pedagogical implications are also discussed.