Assessing the content of learners' compositions is a common practice in second language (L2) writing assessment. However, the construct definition of content in L2 writing assessment potentially underrepresents the target competence in content and language integrated learning (CLIL), which aims to foster not only L2 proficiency but also critical thinking skills and subject knowledge. This study aims to conceptualize the construct of content in CLIL by exploring subject specialists' perspectives on essays' content quality in a CLIL context. Eleven researchers of English as a lingua franca (ELF) rated the content quality of research-based argumentative essays on ELF submitted in a CLIL course and produced think-aloud protocols. This study explored some essay features that have not been considered relevant in language assessment but are essential in the CLIL context, including the accuracy of the content, presence and quality of research, and presence of elements required in academic essays. Furthermore, the findings of this study confirmed that the components of content often addressed in language assessment (e.g., elaboration and logicality) are pertinent to writing assessment in CLIL. The manner in which subject specialists construe the content quality of essays on their specialized discipline can deepen the current understanding of content in CLIL.