When conducting second language (L2) writing experiments with participants of varying proficiency, careful selection of both the test used to estimate participants’ L2 proficiency and the tasks used to elicit writing data from participants is essential. However, L2 writing studies involving learners with varying proficiency rarely discuss the criteria or rationale behind the selection of their research instruments. In this article, we describe how we collected data from a small sample of L1 Dutch learners of Swedish with a wide range of proficiencies and used these to inform our selection of a proficiency test and a set of writing tasks for a research project on the role of proficiency in L2 writing. Each learner completed two proficiency tests, four self-developed writing tasks, and a post-task interview. We used these data to examine the suitability of the two proficiency tests for estimating participants’ L2 Swedish proficiency and the ability of the writing tasks to elicit data from all participants, regardless of proficiency. One proficiency test outperformed the other in terms of efficiency. The writing tasks elicited suitable writing data from all participants, although the interview data also revealed some aspects of the writing tasks that could be improved.