This quasi-experimental non-equivalent comparison group study examines outcomes for participants in eight programs conducting criminal justice diversion for people with co-occurring serious mental illness and substance use disorders compared with jail detainees eligible for diversion, but who were processed through standard criminal justice methods without diversion. Nearly 2000 participants were interviewed at baseline, and 1500 at 3 month and 1300 at 12 month follow-up to baseline. In these interviews, outcome measures of re-arrest, mental health functioning, substance abuse, quality of life, and service utilization were obtained. Those diverted were more likely to have received mental health counseling, mental health medication, and mental health hospitalization than those not enrolled in a diversion program, but were equally likely to have received substance abuse counseling. Overall, the differences in proportions receiving services between the two groups were small, even when these differences were statistically significant. The effect associated with diversion differed somewhat across the individual sites. However, overall cross-site pooled analyses revealed no outcome differences between groups on measures of mental health symptoms, substance use, criminal justice recidivism, or quality of life. Although the immediate benefit of diversion as an access mechanism to community treatment is indicated in pooled cross-site results, such access was driven by more coercive (pre-booking and court) models and results suggest that effecting substantially greater access to services or services use did not occur. The findings also suggest that mental health, substance abuse, and criminal justice outcomes remain dependent on the treatment intervention received, perhaps moderated by type of diversion intervention, rather than on a generic and initial diversion event. Copyright © 2004 John Wiley & Sons, Ltd.