This paper examines the adoption of ISO 14001, which is known as the most famous voluntary environmental program. The data of this paper pertain to Korean [Throughout this paper, Korea refers to the Republic of Korea (South Korea)] firms in manufacturing industries from 1996 to 2011. Event-history modeling to examine firms’ adoption of ISO 14001 finds that both resource-based factors and institutional factors have influenced the diffusion of ISO 14001 in Korea. By exploring time-related effects, I also find that while resource-based factors are important in the early periods of the diffusion, institutional factors become important in the later periods of the diffusion. This confirms the findings of previous studies that a firm’s motivation to adopt organizational policies varies according to different diffusion periods. The paper concludes with a brief discussion of what this study tells us about the institutional context of ISO 14001 in Korea and Asia more broadly. © 2015 Springer Science+Business Media Dordrecht.