Two widely studied language skills in relation to reading comprehension are listening comprehension skill and academic language proficiency. Although their constituent skills and theoretical accounts of how they are related to reading comprehension share a large overlap, they have been studied in separate lines of work. In this study, we investigated the dimensionality of listening comprehension and academic language proficiency tasks, their relations to reading comprehension, and the impact of assessment modality (reading vs. oral language) of academic language proficiency, using data from children in Grade 2 (N = 350). Two cohorts of children from the same schools were assessed on the same set of listening comprehension, word reading, reading comprehension, and academic language tasks. Whereas the first three constructs were assessed in identical manner across the 2 cohorts, academic language tasks were assessed in different modalities (1 cohort in a reading context and the other cohort in an oral language context). Academic language proficiency and listening comprehension skill tasks were best described as having a general oral language construct that captured common variance among all the tasks as well as having specific residual factors. Students' average performance on academic language tasks was lower in the reading context, wherein students' reading skill was also captured beyond the academic "language" proficiency. Across assessment modalities, it was the general oral language construct, not the specific factors, that was reliable, and consistently and most dominantly related to reading comprehension after accounting for word reading. [For the corresponding grantee submission, see ED602242.]