Reading texts may serve as a vital source of informational content and language knowledge for second language (L2) learners, which is known as the dual relevance of L2 reading (Han & D'Angelo, 2009). Text complexity, however, influences the two, determining to a great extent whether the dual outcome is attainable. Drawing on Complex Dynamic System Theory (CDST, de Bot, 2017), the present study conceives text complexity as residing primarily in the interrelationship between language and content complexity and their respective subsystems. To apply this conceptualization to text analysis, this study analyzed language complexity and content complexity of authentic science texts sampled from high school and college textbooks on four different subjects. Results show that college texts in general exhibited greater language and content complexity than high school texts, especially in terms of sentence length and the use of complex nominals. Aside from this emerging pattern, variability characterized the magnitude of difference in complexity and the manner in which the texts differed. Overall, the findings from this study attest to the mutual relationship between language complexity and content complexity embedded within authentic texts.