This thesis attempts to examine the effect of varying levels of staff cohesion, a key process variable in the residential treatment of disturbed adolescents, on the quality of treatment environment. In order to do this the staff from each of the three residential cottages which constitute the Easton residential unit were given the Seashore Index of Group Cohesiveness (1954). It was then possible to delineate three levels of staff coehsion, high, medium, and low. The same staff groups also completed a treatment environment scale, namely the Community Orientated Programs Environment Scale (Moos, 1974), Forms R (real) and I (ideal). The hypothesis here was that as staff cohesion rose so the perceived quality of the treatment environment would rise. The clients from the cottages also completed the environment scale, the hypothesis being that their perceptions of the environment would be more positive in the cottage containing the most cohesive staff group. Client behaviours in the low, medium and high cohesion cottages were also observed. The study showed that there were no significant differences between the staff groups concerning their view of the ideal treatment environment, but the level of cohesion did have an effect on the real treatment environment, as perceived by the staff, specifically on the variables Support, Personal Problem Orientation, Autonomy, Practical Orientation and Staff Control. The number of locked room hours for clients and the number of violent incidents also seemed to follow the cohesion factor pattern. However, the clients' perceptions of their environment did not vary significantly according to cohesion level. Some descriptive data comparing the factors in the Easton treatment environment to the norms for such programs were also included. Implications for the planning and evaluation of such treatment programs have been outlined as well as suggestions for further research.