Voluntary environmental programs are codes of progressive environmental conduct that firms pledge to adopt. This paper investigates whether ISO 14001, a voluntary program with a weak sword--a weak monitoring and sanctioning mechanism--can mitigate shirking and improve participants' environmental performance. Sponsored by the International Organization for Standardization (ISO), ISO 14001 is the most widely adopted voluntary environmental program in the world. Our analysis of over 3,000 facilities regulated as major sources under the U.S. Clean Air Act suggests that ISO 14001-certified facilities reduce their pollution emissions more than non-certified facilities. This result persists even after controlling for facilities' emission and regulatory compliance histories as well as addressing potential endogeneity issues between facilities' environmental performance and their decisions to join ISO 14001.