Parent-infant interactions provide essential contexts in which infants learn strategies for social functioning and regulating arousal. This study explored two structural qualities of parent-infant interactions, dyadic synchrony and flexibility, thought to reflect mutual regulation. The study further explored relations among these structural constructs and contextual factors including marital satisfaction, parental depressive symptoms, parenting stress, and parent and infant affect in the interaction. Mothers, fathers, and their 6-month old male and female infants (N = 164) took part in the Still Face Paradigm. Parents provided self-report of marital satisfaction, depressive symptoms, and parenting stress. Results suggest that synchrony and flexibility represent related but independent constructs. Flexibility and synchrony were associated with parent and infant affect in the interaction; results differed slightly depending on the interactional context (mother/father, face-to-face/reunion). Mother-infant flexibility in the reunion episode was related to maternal depressive symptoms and report of parenting stress. Maternal positive affect in the reunion interaction was associated with maternal report of marital satisfaction; whereas infant negative affect expressed in father-child reunion interactions was associated with paternal marital satisfaction. Results illuminate the distinctions and similarities between flexibility and synchrony, and the relations among flexibility in parent-infant interactions and contextual factors.