Voluntary health and safety programs are now a common programmatic approach that health and safety regulators use to promote health and safety in the workplace. These programs may include education and training, performance evaluation and OHS audits. Audit-based voluntary programs that provide firms meeting specific OHS audit criteria with rebates on worker’ compensation premiums are now in place in many Canadian provinces. In British Columbia, in 2003, WorkSafeBC introduced an OHS certificate of recognition program that is now available across multiple industrial sectors including forestry, construction, manufacturing and transportation and warehousing. This presentation provides results of an impact evaluation of OHS certification on firm injury rates during the years 2003 to 2012. It highlights the theoretical and practical challenges of program evaluation where program participation is voluntary. Overall, using observational research methods, firms that met the OHS certification standard had, on average, a 12% to 17% lower compensation claims rate than similar non-certified firms. This finding is interpreted that the OHS certification audit process in British Columbia is effective at identifying firms with lower work injury risk.