Mental visualization of diagrammatic representations is presumed to be a critical strategy for learning and problem solving, particularly in the sciences. However, little is known about how students employ visualization on genuine scientific tasks. The present study describes undergraduate chemistry students’ use of visualization for problem solving using think-aloud protocols. The analysis suggests that students’ reasoning is heavily guided by the form of the molecular diagrams given in the task and self-generated inscriptions. Visualization strategies appeared critical for task that required representation translations.