<p>Mutual engagement, the dynamic process through which collaborators reciprocally take up and sustain one another’s ideas and actions, is crucial to collaborative problem solving (CPS). However, existing research has yet to fully specify concepts or methodologies needed to capture these dynamic characteristics. This gap highlights the need to examine how these patterns evolve across different CPS phases to inform more sophisticated instructional strategies that enhance collaborative learning. This exploratory study integrates multimodal and content analyses to examine phase-sensitive patterns of mutual engagement in small teams. In total, 28 college students participated in video-recorded CPS activities across four distinct phases. The findings revealed that high-performing teams displayed structurally complete elaborative sequences, in which invitations to contribute were taken up, elaborated, and reciprocated. Low-performing teams, by contrast, exhibited fragmented sequences that failed to return to elaboration. These interactional differences co-occurred with distinct multimodal signatures. High-performing teams exhibited greater interest, less frequent neutral emotions, and early posture synchrony, patterns that were especially pronounced during the ideation phase. Conversely, low-performing teams showed lower interest, persistent neutral emotions, and late, reactive posture synchrony. The findings elucidate the socio-cognitive characteristics of mutual engagement and demonstrate the potential for integrating emotional and behavioral indicators for a richer understanding. These insights can inform the design of instructional scaffolding and phase-sensitive support systems to enhance successful collaborative learning.</p>