The assessment of task-generated cognitive demands has been receiving increasing attention in task complexity research. However, scant attention has been paid to assessing cognitive demands when task complexity is manipulated along both resource-directing and resource-dispersing dimensions. To address this gap, the present study aimed to investigate the relative effects of reasoning demands and prior knowledge on cognitive demands in L2 writing. Eighty-eight EFL students completed two letter-writing tasks with varying reasoning demands under one of two conditions, that is, either with prior knowledge available or without prior knowledge available. Cognitive demands were assessed by the post-task questionnaire, the dual-task method and the open-ended questions. The results revealed that reasoning demands and prior knowledge were strong determinants of cognitive demands, which provided empirical evidence for Robinson’s Cognition Hypothesis. Moreover, the post-task questionnaire, the dual-task method and open-ended questions were found to assess distinct aspects of cognitive demands, which highlighted the importance of data triangulation in exploring task complexity effects. The study provides language teachers and assessors with implications for task design and implementation. • How reasoning demands and prior knowledge affect cognitive demands was underexplored. • Cognitive demands were assessed by both quantitative and qualitative methods. • Findings supported some assumptions underlying Robinson’s framework. • The independent measures assessed distinct aspects of cognitive demands.