Little is known about the legislative process and in particular how this relates to environment in Latin America. This article attemps to partially close this gap by discussing the dynamics of congressional environmental politics in Chile under former President Lagos (2000-2006). At first, this article discusses the role of the executive branch in the legislative process, congressional authority and environmental policy. This sometimes uneasy relationship is explored, and its consequences over environmental politics are discussed. The second part of the article attempts to explain legislator's choices in the area of environmental policy, developing and testing four main hypotheses that eventually help to explain why legislators support/reject laws of positive/negative environmental relevance. Finally, this article draws general conclusions on Chilean congressional politics and environmental policy, to then propose some recommendations on how to improve the process of creating environmental policy in Congress. © 2010 Taylor & Francis.