A protocol analysis of second language (L2) writing of adult speakers of Japanese, English, and Spanish observed how language switching (first language (L1) switching use in L2 writing) was affected by L2 proficiency, task difficulty, and language group. Less proficient L2 learners switched to their L1 more frequently than more advanced learners, and that more difficult tasks increased duration of L1 use in L2 writing. (Author/VWL)