Arsenic-bearing wastes are common by-products of mineral processing and smelting. As arsenic is well known to be highly toxic, it is important that these wastes should be treated in an ecologically acceptable manner. As(III) is the species of principal concern. One treatment system uses Portland cement ar; a containment and chemical immobilization matrix. A review of the literature revealed the solid phases of relevance, and solubility data were critically evaluated. Based upon these data, computer-based modelling was used to identify stable phase assemblages in the CaO-SiO2-As2O3-H2O system at 25 degrees C and calculate equilibrium solubilities. An experimental program was undertaken to verify the modelling predictions within the CaO-As2O3-H2O sub-system, and experimentally-obtained solubility data were, in turn, used by the modelling program. Solubility data thus derived were critically compared with those of the literature and although there are some differences between the data sets recast in the form of phase diagrams, the two are numerically in good agreement. The solubility of As is limited by reaction with calcium hydroxide and C-S-H gel to similar to 0.0007 mmol kg(-1), or approx. 0.075 mg l(-1) as arsenite (AsO2-), by formation of CaAsO2OH. (C) 1997 Elsevier Science Ltd.