Focusing on the relationship between linguistic, cognitive, socioemotional factors in writing English for academic purposes (EAP), this study investigated whether topic familiarity as an important cognitive factor of task complexity influences different levels of emotionality and linguistic complexity in EAP writing and whether there are relationships between emotionality and linguistic complexity. To do so, 64 international graduate learners enrolled in EAP writing courses participated in the present study. Each wrote on familiar and unfamiliar topics determined via a questionnaire at the onset of the study. Their writings were then measured for textual emotionality and linguistic complexity using automatic assessment tools. Results showed that EAP writings differed systematically in terms of both emotionality and linguistic complexity due to the influence of topic familiarity. Unfamiliar topics led to writing performance with a significantly higher level of emotional negativity and significantly lower linguistic complexity levels as compared to familiar topics. A follow-up correlation analysis also revealed significant relationships between emotionality and linguistic complexity measures, indicating complex interactions between linguistic and socioemotional factors. Implications of these findings are discussed relative to deploying writing topics with varied levels of cognitive complexity for encouraging classroom engagement and improving L2 learners' writing performance by effective task sequencing.