An immobilization process is described which is based on sulphur (instead of cement) as a binding agent for the treatment of hazardous waste materials. Elemental sulphur is able to bind chemically metals such as mercury and, to a lesser extent, lead as metal sulphides. Furthermore, sulphur forms a compact solid mass that has a low permeability, and consequently can be leached only to a limited extent. As a result of the addition of sulphur the compressive strength of the pelleted waste materials tested also improves considerably. The waste materials were residues from the incineration of municipal solid waste. Sulphur can be processed in more or less the same way as asphalt, as it is processed at about 130°C. The amount of sulphur that was added ranged from 25 to 35%. Details of the leaching properties of sulphur-bound waste materials are given together with an outline description of the process. The results suggest that, with some additional treatment, the process may produce artificial gravel suitable for building purposes.