A lack of familiarity with the lexical, grammatical, and discursive features associated with academic language presents comprehension challenges for many adolescent readers, especially those with still-developing English language skills. Although many studies focused on reading comprehension in content areas, the prevalent use of standardized tests combining narrative and expository readings across academic domains undermines our understandings of the specialized language and skills required for comprehension in each particular discipline. In this study, we examined contributions of domain-general and domain-specific language skills needed for reading in science, a linguistically and cognitively demanding subject area. The sample included 241 Grade 8 students, 64 of whom were emergent English speakers (current ELs). Proficient English speakers included 118 former ELs. Our statistical modeling results explained 80% of the variance in students' science reading comprehension and science vocabulary scores. Mediated effects of morphosyntactic awareness and a direct effect of science vocabulary on reading comprehension of physical sciences texts were supported. Support is also offered for an appropriate measurement specification for the construct of morphosyntactic awareness and the appropriateness of using definition (breadth) and picture (depth) match tasks for assessing science vocabulary. Less variance was explained in students' morphosyntactic awareness, a variable that needs further examination.