Open AccessConsidering the growing interest in task-based language teaching, classroom-based research that investigates the effects of task complexity on L2 development is needed. Despite the inclusion of task reasoning demand (TRD) as a dimension of task complexity in Robinson’s Cognition Hypothesis (2007), there is insufficient classroom-based research that investigates the language learning outcomes that may occur as a result of engaging in tasks of differing reasoning demands in a variety of task conditions. This study aims to fill in some of the gap by identifying the main and interaction effects of task reasoning demand and individual versus dyadic task conditions (TC) on the grammatical accuracy and syntactic complexity of learner written output. Modified versions of the dictogloss task and the opinion-gap task were used to provide a relatively high reasoning demand task (+TRD) and a relatively low reasoning demand task (-TRD) to the learners respectively. A repeated-measures design was used with 76 participants consisting of 18 year-old learners in a public secondary school randomly assigned into four groups. Data were analysed using descriptive statistics and repeated-measures ANOVAs. Results indicated that both TRD and TC had significant main effects on grammatical accuracy. Also, TRD and TC had significant main and interaction effects on syntactic complexity. The results point to differential effects of using tasks of high and low reasoning demand in dyadic and individual task conditions. The results have pedagogical implications on task design and task selection to elicit higher rates of grammatical accuracy and syntactic complexity in learner written output.