This study investigated the influence of general vocabulary knowledge, science vocabulary knowledge, and text based questioning on the science reading comprehension of three types of students who varied in their English language proficiency. Specifically, grade 5 English-Only speakers, English Language Learners in the United States, and students learning English as a Foreign Language in a Spanish-speaking country were compared across the aforementioned variables to examine their relationship to reading comprehension. Findings indicated that general vocabulary, science vocabulary, and questioning contributed significant variance to the explanation of science reading comprehension. However, no particular variable was found to interact with language proficiency in their contribution to science reading comprehension. Findings are discussed in terms of the roles that each variable could play in content area comprehension. Implications for content-area literacy instruction of second-language students are considered.