Solidification/stabilisation of an industrial waste material containing large amounts of arsenic, as As2O3 (32wt%), was studied. The waste was treated using solidification/stabilisation (S/S) with cement (Ecobind50), lime and blast-furnace slags (Betogrid) in order to reduce the leachability of arsenic. In order to optimize the procedure for S/S, 10 different S/S samples, differing in amount and combination of the binder materials were prepared. On these samples, extraction tests were performed, showing that the arsenic concentration in the leachate is correlated with the pH and the calcium concentration due to the formation of slightly soluble CaHAsO3. A semi-dynamic leach test was performed in order to obtain quantitative information on the long term leachability of the contaminants (As, Sb, Pb) from monolithic S/S samples. A linear relationship between the cumulative fractions released and the square root of the leach time was observed, indicating that diffusion is the release mechanism. From the diffusion coefficient, the influence of the binder materials on the leachability of the contaminants could be deduced. All the elements considered have a low mobility in all samples. Ca has the highest mobility followed by As and Sb, whereas Pb has the lowest mobility. It appeared that increasing the amount of calcium in the S/S samples lowers the leachability of arsenic, addition of lime being more effective than addition of cement. This conclusion is in good qualitative agreement with the extraction tests. (C) 1997 Elsevier Science Ltd.