A measure of group atmosphere was completed 1 month before the end of treatment by both the group leaders and the group members of 12 sexual offender treatment groups at 6 different medium-security prisons that used the CBT group treatment standardized for sexual offenders in British prisons. The Group Environment Scale (Moos, 1986) was used to measure the following aspects of group processes: relationships within the group, personal growth of members, and the structure of the group. Preintervention and postintervention measures were used to assess treatment-induced change in pro-offending attitudes. In order to assess whether group process had an impact upon treatment outcome, correlation analyses were performed between the variables identified as being significantly different between the various groups and the treatment outcome data. The findings suggest that group involvement by members and their commitment to and concern/friendship for each other, plus the extent to which freedom of action and expressions of feelings are encouraged within the group, are significantly related to treatment outcome. The finding that the encouragement of emotional expressiveness facilitated positive change in treatment is consistent with observations reported in the general psychotherapeutic literature. 4 tables and 38 references