The stabilized filter cake produced at a commercial stabilization/solidification plant, which treats a wide variety of inorganic solid wastes and effluents, has been characterized, and subsequently solidified by adding increasing amounts of cement and fly ash. This stabilized waste interacts with the cementitious additives so that the normal products of hydration are no longer produced. A solidified material is, however, still formed, and ettringite is the main crytalline product. The degree of chemical containment has been determined by completing single extraction batch leach tests under increasingly aggressive conditions. Results indicate that hazardous heavy metals are well retained by the solidified wastes, while they are readily leached from the stabilized filter cake. The concentrations of metals released from the stabilized waste and the solidified materials exhibit the same behaviour as a function of leachate pH. This demonstrates that a major effect of the cementitious additives is to change the composition of the leaching fluid. The addition of solidified wastes containing increasing amounts of cementitious additives to an acidic leachant causes Ca2+ to leach at a concentration determined by the acidity of the leachant. Mixes containing low additions of cementitious additives cannot release sufficient Ca2+, the leachate remains acidic, and heavy metals present in the waste are able to leach.