Literature has shown the importance of mentalizing techniques in symptom remission and emotional understanding; however, no study to date has looked at the dynamic relations between mental state talk and affect regulation in the psychotherapy process. From a psychodynamic perspective, the emergence of the child’s capacity to regulate affect through the therapist’s reflection on the child’s mental states is a core aspect of treatment. In an empirical investigation of 2 single cases with separation anxiety disorder, who were treated in long-term psychodynamic play therapy informed with mentalization principles, the effect of therapists’ and children’s use of mental state talk on children’s subsequent capacity to regulate affect in play was assessed. One case was a positive outcome case, whereas the other did not show symptomatic improvement at the end of treatment. Children’s and therapists’ utterances in the sessions were coded using the Coding System for Mental State Talk in Narratives, and children’s play was coded by Children’s Play Therapy Instrument, which generated an index of children’s “affect regulation.” Time-series Granger Causality tests showed that even though both therapists’ use of mental state talk significantly predicted children’s subsequent affect regulation, the association between child’s mental state talk and affect regulation was only supported for the child who showed clinically significant symptom reduction. This study provided preliminary support that mental state talk in psychodynamic psychotherapy facilitates emotion regulation in play.