The aim of this paper is to investigate the role of supply chain collaboration between buyers and suppliers in the reverse flow of a system of circular supply chains. The empirical context is Swedish unalloyed steel recycling, i.e. recycling of steel with a low proportion of additional components. The paper applies a framework on mechanisms of coordination and supply chain flows to discuss the underlying rationale for how the Swedish steel recycling industry organizes coordination. At the activity level, it is shown how the inherent uncertainty of reverse flows is handled through voluntary cross-industrial cooperation and standardization.