Most State and local criminal justice systems offer criminal justice or jail diversion programs for certain classes of offenders who have committed less serious offenses and suffer from mental illness or substance abuse problems. Although these programs are widely employed, there is little evidence on the cost-effectiveness of these jail diversion programs. Drawing on data from four jail diversion sites, the authors offer one of the first cost-effectiveness studies of jail diversion programs. The eligible population of the four sites included people with co-occurring serious mental illness and substance abuse or dependence disorders. Four of the sites were post-booking programs and one was a pre-booking program. Costs that were incurred by all publicly funded agencies directly involved with the jail diversion programs were taken into account, as well as labor, materials, rent, utilities, maintenance and supplies, capital costs, and administrative overhead. Effectiveness outcome measures included self-report data of criminal behavior, quality of life, substance use, and mental health status. Results are presented separately for each site and generally revealed that diversion resulted in lower jail costs. However, the treatment domain drove the analysis, revealing that the amount and unit cost of treatment was associated with higher diversion costs. The availability of less costly treatments may be crucial to controlling the costs of pre-booking jail diversion programs. Limitations of the study included incomplete cost and effectiveness data due to limited resources. Future research should focus on what offenders are being diverted from, what they are being diverted to, and the timing of the diversion process. Tables, notes, references