PROBATIONERS WHO MET THE ELIGIBILITY CRITERIA WERE RANDOMLY ASSIGNED TO EITHER VOLUNTEER SUPERVISION (VS) OR PROFESSIONAL SUPERVISION (PS). THE VS SAMPLE SIZE WAS 94 OVER THE 36-MONTH STUDY, AND THE PS SAMPLE SIZE WAS 96. THE PROCESS MEASURES USED INCLUDED ATTITUDE AND PERSONALITY SCALES, PARTICPANT REPORTS, RATING BY PRIVILEGED OTHERS, AND BEHAVIORAL MONITORING OF AUDIO-TAPED SUPERVISORY SESSIONS. THE OUTCOME INDICES INCLUDED ATTITUDE CHANGE MEASURES AND IN-PROGRAM RECIDIVISM. THE HYPOTHESIS THAT VS AND PS EFFECTIVENESS WOULD DIFFER ACCORDING TO HOW STRUCTURAL VARIATIONS IN THE ROLES WAS SUPPORTED. VOLUNTEER SUPERVISION EMPHASIZED RELATIONSHIP QUALITY, WHILE PROFESSIONAL SUPERVISION EMPHASIZED THE AUTHORITY INHERENT IN THE PROBATION ORDER. VOLUNTEERS ENGAGED CLIENTS IN MORE OVERALL PROBLEMSOLVING THAN DID THE PROFESSIONALS, HOWEVER, PROFESSIONAL PROBLEMSOLVING GUIDANCE WAS ON MORE COMPLEX LEVELS. VS WAS SIGNIFICANTLY MORE EFFECTIVE THAN PS WITH THE HIGHER RISK CLIENTS (YOUNGER AND LESS SOCIALIZED), PARTICULARLY WITH THOSE CLIENTS SHOWING PRETEST EVIDENCE OF INTERPERSONAL SKILLS. ON THE BASIS OF A DISCRIMINANT ANALYSIS OF THE TAPE-BASED PROCESS MEASURES, EFFECTIVE SUPERVISION BY BOTH VOLUNTEERS AND PROFESSIONALS WAS CHARACTERIZED BY A HIGH LEVEL OF ATTENTION TO THE PROBATION ORDER, MODELING AND DIFFERENTIAL REINFORCEMENT OF PROSOCIAL EXPRESSIONS, PROBLEMSOLVING WITH A COMMUNITY FOCUS, FREQUENT FRIENDLY EXPRESSIONS, AND LOW RATES OF EMPATHETIC LISTENING. SUPERVISION CHARACTERISTICS APPARENTLY UNRELATED TO REDUCED RECIDIVISM WERE PROBLEMSOLVING WITH A RECREATIONAL FOCUS OR A PERSONAL-EMOTIONAL FOCUS, ADVOCATE-BROKER ACTIVITY, OFFICER SELF-DISCLOSURE, AND LENGTH OF INTERVIEWS. ON THE BASIS OF THE FINDINGS, A PRINCIPAL RECOMMENDATION IS THAT VOLUNTEER ACTION PROGRAMS BE STRUCTURED AS COUNSELING PROGRAMS RATHER THAN AS COMPANION OR RECREATIONAL ACTIVITIES. VOLUNTEERS ASSIGNED ASSISTANT COUNSELOR ROLES ARE ADVISED TO BE INTERPERSONALLY SENSITIVE AND FLEXIBLE WHILE BEING PROSOCIAL AND ANTICRIMINAL IN ORIENTATION. TABULAR DATA AND REFERENCES ARE PROVIDED. (RCB)