As an initial step toward understanding which features of academic language make science-based expository text difficult for students with different English language proficiency (ELP) designations, this study investigated fifth-grade students' thoughts on text difficulty, their knowledge of the features of academic language, and the relationship between academic language and reading comprehension. Forty-five fifth-grade students participated in the study; 18 students were classified as English language learners (ELLs) and 27 students were fluent-English speakers. Participants read two science passages, answered comprehension questions, and engaged in a retrospective interview which probed their knowledge on the academic language features of vocabulary, grammar, and discourse. Qualitative analysis was used to code students' thoughts about the challenges to reading comprehension and to identify the challenges that were related to academic language. Quantitative analyses were conducted to examine whether students' knowledge of academic language features and reading comprehension differed by students' ELP designations, as well as to investigate the relationship between students' knowledge of academic language features and reading comprehension. Results for the qualitative analysis revealed that students found difficult vocabulary, reading abilities, and prior knowledge as the greatest challenges to comprehending the science passages. Results from the quantitative analyses indicated that ELL students' knowledge of academic vocabulary, grammar, discourse knowledge, and reading comprehension (as measured by multiple-choice questions) were significantly lower than the fluent-English speaking students. The results also indicated that vocabulary, not grammar or discourse features, was significantly related to students' comprehension scores. The results have implications for understanding the features of academic language that influence students' comprehension of expository texts in science. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]