Most of the city of Uppsala rests on postglacial clay with a varying content of sulphur. The sulphur occurs naturally in the clay in reduced form as sulphide and the clay is for that reason usually called ?sulphide clay?. Excavation during construction activities gives rise to large amounts of excavation material, of which the sulphide clay is a considerable part.When the clay is exposed to air and water, an oxidation of the sulphides occurs which may produce an acid leachate and the mobilisation of heavy metals bound in the clay or underlying material. The aim of the work has been to improve our understanding of the behaviour of sulphide clays and produce a basis for risk assessment in the future handling of excavation material with high sulphide content.