The present study explores the use of conjunctions in simple versus complex argumentative tasks performed by second language (L2) learners as a specific measure for the amount of reasoning involved in task performance. The Cognition Hypothesis (Robinson, 2005) states that an increase in cognitive task complexity promotes improvements in L2 performance. This effect should become particularly visible when task‐specific performance measures are used (Robinson & Gilabert, 2007). This article evaluates these claims by investigating the oral performance of 64 L2 learners on cognitively simple, as compared with cognitively complex, oral argumentative reasoning tasks. The analysis focuses first on the overall frequency and occurrence of conjunctions. Next, 5 conjunctions that are considered to be highly task‐relevant are examined more closely. Results are discussed in light of the speech production of 44 native speakers who performed the same tasks under the same conditions. The discussion addresses implications of the findings for the cognitive approach to task‐based L2 research in light of Robinson’s (2005) Cognition Hypothesis. From the standpoint of research methodology it highlights the benefits of native speaker data as a baseline for comparison.