This study presents a coding system for observation and monitoring of changes in the interactive behaviour between patient and therapist during music therapy sessions. The coding scheme was developed from a psychodynamic framework and mainly consists of four behavioural classes: Verbal Communication, Nonverbal Communication, Countenance, and Sonorous Musical Communication. The 15 minutes in the middle of each videotape concerning the first active music therapy session--based on the sonorous musical improvisation--were coded. Subjects were children (4 boys; 3 girls) ages 3 to 10 years (M age = 6.3), diagnosed with Pervasive Developmental Disorder, and seven therapists. The method for data collection was continuous recording, applied through The Observer Video-Pro 5.0. For the reliability indexes there was a substantial agreement between assessments by video raters.