Many U.S. cities face the question of how to redevelop vacant post-industrial land, or brownfields. The city of Chicago responded to this issue with The Calumet Open Space Reserve (COSR), which was established in 2005 and set aside nearly 4,000 acres of land in Southeast Chicago for future open space redevelopment. The COSR marks a shift in Chicago’s brownfields policy from a focus on commercial and industrial revitalization to ecological preservation and open space provision. Drawing on existing plans, studies, and other archival documents from the Chicago Department of Planning and Development, I argue that the COSR was successful in transforming once vacant contaminated land into publicly accessible open space, in part due to the ecological and industrial context of the Calumet Region. Using a framework of socioenvironmental succession and accumulation by degradation, I argue that the COSR serves to decouple environmental degradation from industrial expansion. The implementation of this plan marked a shift in open space planning and management that allowed for the coexistence of industry and open space and proposed a new model for greenspace in Chicago and urban areas more broadly.