Abstract: This is a qualitative study that looks into theinterplay of crucial variables in the successful comprehension of two scientific texts of different topic familiarity and syntactic difficulty. Respondents were three EFL undergraduates with some prior knowledge on the two scientific texts but different levels of English proficiency. The aim was to see how they negotiate the two texts given their current level of English proficiency and prior knowledge. For this purpose, think aloud protocols and structured interviews were conducted to determine their strategy choices. The study shows that even though L2 proficiency and prior knowledge of the science topic are important contributors, strategy choices are instrumental to successful comprehension. While lower cognitive strategies are used to get the meaning from the written texts, it is crucial to use higher cognitive strategies to complete the understanding.