Second language (L2) learners often resort to machine translation (MT) to help them during the writing process, which is why it is important to understand how L2 learners use MT tools during L2 writing and whether and how using MT impacts the writing product and process of L2 students. However, it is still unclear how L2 proficiency impacts the way L2 learners use MT tools during L2 writing. Moreover, it remains to be determined how different ways of MT use affect the L2 writing product across proficiency levels. Further research, taking more measures for variation in MT lookup behavior into account, in combination with L2 proficiency, is needed to fully understand how access to MT during writing affects the L2 writing product. Lastly, the effects of MT use on the L2 writing process are still underresearched, in particular in terms of the cognitive processes underlying L2 learners' online behavior. Further research, in which online and introspective data of more participants are triangulated, is needed to fully understand how MT use affects the L2 writing process. This project addresses these research gaps by collecting and analyzing keystroke logging, screen capture, eyetracking, stimulated recall and product data of L2 learners responding to writing prompts in two conditions: one with access to MT, and one with access to an online bilingual dictionary instead.