We observed mutual smiling episodes (MSEs) during therapist-couple triadic interaction as a key element of affective exchanges that serve to regulate the therapeutic relationship. Based on a functional perspective, we developed a new rating scale, the MSE Coding System (MSE-CS) that allows us to distinguish between four different MSEs, which correspond to four social functions: supporting mutual binding, sharing miseries, repairing, and confronting. MSEs were analyzed in a sample of eight therapist-couple triads. Therapeutic alliance and marital adjustment were also measured. The instrument (MSE-CS) demonstrated satisfactory inter-rater reliability, and initial indications of validity are promising. Results showed that number of MSEs was positively correlated with triadic therapeutic alliance. Types of MSEs were also distributed differently depending on the level of triadic therapeutic alliance.