We describe the use of think alouds to examine substantive processes involved in performance on a formative assessment of computational thinking (CT) designed to support self-regulated learning (SRL). Our task design model included three phases of work on a computational thinking problem: forethought, performance, and reflection. The cognitive processes of seven students who reported their thinking during all three phases were analyzed. Ratings of artifacts of code indicated the computational thinking problem was moderately difficult to solve (M = 15, SD = 5) on a scale of 0 to 21 points. Profiles were created to illustrate length and sequence of different types of cognitive processes during the think-aloud. Results provide construct validity evidence for the tasks as formative assessments of CT, elucidate the way learners at different levels of skill use SRL, shed light on the nature of computational thinking, and point out areas for improvement in assessment design.