This exploratory study focused on characterizing problem-solving situations associated with spontaneous metacognitive activity. The results came from connected case studies of a group of 3 purposefully selected 9th-grade students working collaboratively on a series of 5 modeling problems. Students' descriptions of their own thinking during small-group mathematical modeling, elicited during video-stimulated interviews, were analyzed to identify and characterize social-based and self-based contexts associated with metacognitive activity coded as awareness, regulatory, and evaluative. Three characterizations of the social-based contexts and 3 different characterizations of the self-based contexts emerged. Among social-based contexts, those characterized as interpreting diverse perspectives were significantly more frequent than those characterized as seeking mathematical consensus. Also, those characterized as engaging in explanations were significantly more frequent that those characterized as seeking mathematical consensus. Among self-based contexts, those characterized as seeking personal satisfaction were significantly more frequent than those characterized as making experience-based quantitative judgments as well as those identified as using personal projections. (Contains 8 tables and 1 footnote.)