Increasing attention has been paid in recent years to the ways writing may engage adolescents in higher levels of epistemic complexity (i.e., postulating causes, reasons and other relations or theories related to scientific phenomena), yet in secondary science classrooms, writing has primarily been used for assessing students' content knowledge. Embedded in a larger national study of secondary writing in the United States, this study investigated the qualities of science writing samples collected from 33 adolescents attending schools identified for exemplary writing performance. We asked: How is epistemic complexity reflected in adolescents' writing?; How does the level of epistemic complexity differ by adolescents' language background, grade level, and school context?; What is the nature of the relationship of types of writing and higher or lower levels of epistemic complexity? We found the majority of writing adolescents produced did not show evidence of high levels of epistemic complexity. Notable exceptions were reading reflections and lab reports. Implications for adolescent science writing instruction are discussed in light of higher standards for disciplinary writing in secondary schools.