Through behavioural analysis, this study investigated the social-motivational aspects of musical interaction between the child and the therapist in improvisational music therapy by measuring emotional, motivational and interpersonal responsiveness in children with autism during joint engagement episodes. The randomized controlled study (n = 10) employed a single subject comparison design in two different conditions, improvisational music therapy and toy play sessions, and DVD analysis of sessions. Improvisational music therapy produced markedly more and longer events of 'joy', 'emotional synchronicity' and 'initiation of engagement' behaviours in the children than toy play sessions. In response to the therapist's interpersonal demands, 'compliant (positive) responses' were observed more in music therapy than in toy play sessions, and 'no responses' were twice as frequent in toy play sessions as in music therapy. The results of this exploratory study found significant evidence supporting the value of music therapy in promoting social, emotional and motivational development in children with autism.