When a fly ash waste material from a copper refining process containing large amounts of AS(2)O(3) is solidified using cement and lime, the arsenic concentration in the leachate can be lowered to ca. 5 mg/l in a saturated solution of Ca(OH)(2). It is shown that the decrease of the concentration in the leachate, mainly of As(III), is due to the formation of insoluble CaHAsO3 in the leachate in the presence of Ca(OH)(2). This method is compared with a method whereby use is made of oxidation of the waste before solidification to oxidise As(III) to As(V) using H2O2. The arsenic concentration in the leachate of the extraction test of an oxidised S/S sample was lowered to ca. 0.5 mg/l, a factor of 10 below the one for a non-oxidised sample. It is shown that the decrease of the concentration in the leachate mainly of As(V) is due to the formation of insoluble Ca-3(AsO4)(2) in the presence of Ca(OH)(2). Extensive use was made of the speciation program MINTEQA2, to clarify the immobilisation of arsenic. (C) 2002 Elsevier Science Ltd. All rights reserved.