This article reports the findings of part of a major study exploring the disciplinary writing processes and perceptions of 15 Chinese graduate students in sciences and engineering at a major Canadian university. The findings relate to the thinking languages of the participants in writing disciplinary assignments. The study reveals that whether an L2 writer thinks in L1 or L2 may not depend on one factor as proposed in earlier studies (Friedlander, 1990; Qi, 1998), but on a number of factors including the language of knowledge input, the language of knowledge acquisition, the development of L2 proficiency, the level of knowledge demands, and specific task conditions. It is the interplay among these (and possibly other) factors that determines the writer's choice of the thinking language, which may switch back and forth between L1 and L2. Further, although translation may be a positive strategy for a student with limited L2 proficiency, it may gradually phase out as the student thinks more in L2 and writes L2 in approximation to the language of native writers. Thus a thinking language continuum may exist along which the use of translation varies.