An industrial waste containing large amounts of arsenic (42 wt %) was studied. The waste was treated using solidification/stabilisation sation technology (SIS) with cement and pozzolanic materials. Addition of lime appeared to be the major factor in reducing the concentration of arsenic in the leachate from the S/S material through the formation of a slightly soluble calcium-arsenic compound in the leachate. To investigate the mechanism of leaching from the SIS waste, semi-dynamic leach tests were performed (ANSI/ANS 16.1), but no direct conclusions concerning the release mechanism could be drawn from these tests, as equilibrium was reached during the intervals of static leaching of which the semi-dynamic leach test consists. Therefore, static leach tests were carried out that gave much information about the behaviour of arsenic in the leachate. Moreover, it is shown that arsenic in the leachate is mainly present as As(III) and evidence is presented for the formation of a CaHAsO3-precipitate. It is concluded that semi-dynamic leach tests may give inaccurate information, if no preliminary study is made of the SIS waste-leachate equilibrium.