Due to its authenticity as an academic writing task, integrated writing assessment has become widely used for assessing the writing ability of English for academic purposes (EAP) students (Plakans & Gebril, 2017). However, apart from validation studies focusing on a common set of standardized test rubrics (e.g., Chan, Inoue, & Taylor, 2015), little research has explored the construct of integration or how to assess it effectively in EAP contexts. To provide second language (L2) writing researchers and practitioners with an empirically based model of source integration, this study explored EAP instructors' orientations when assessing integrated essays and investigated the relationship between instructor ratings and textual measures of source use. Six experienced EAP instructors first evaluated two sample argumentative essays written by undergraduate English L2 students and provided comments through a stimulated recall interview. Next, they evaluated 48 additional argumentative essays using an analytic rubric that included dimensions of source use and provided comments for each essay. Finally, the essays were analyzed for linguistic and rhetorical features associated with source integration. Triangulation of data sources revealed that the instructors oriented to three aspects of source integration: using source information to support personal claims, incorporating source-text language in students' own words, and representing source content accurately. Their ratings were correlated with text-based measures of students' source use. Implications for teaching and assessing integrated writing in EAP settings are discussed.