Abstract Sustainable development has become a keyword and a guiding principle for metropolitan and urban planning. This paper treats sustainability from a procedural perspective and investigates the following questions: can public participation be a tool, first for defining the content of urban sustainability, then for implementing it? How is this concept mobilized in public debate? To look into the questions, an analysis of two major public debates held in relation to urban planning in Montreal is conducted: first, the public hearings held by the OCPM (Office de consultation publique de Montreal) in 2004 for the new city wide master plan; then the ones held by the South-West Borough in 2008 for the Griffintown development project. The paper contrasts the two cases as to their nature, the procedures involved, who were the public participants with issues raised, and examines linkages between them, both expected and actual, to conclude with more theoretical comments about urban sustainability and "the engineering of public participation."