When a waste fly ash, containing large amounts of As 2 O 3 , is solidified using cement and lime, the arsenic concentration in the leachate (extraction test DIN 38 414 S4) is determined by the solubility of CaHAsO 3 and can be lowered to a value of ca. 5 mg/l, in a saturated solution of Ca(OH) 2 . One of the criteria for landfilling of hazardous waste is, however, that the arsenic concentration in the leachate must be lower than 1 mg/l. In this paper, it is shown that oxidation of the waste before solidification, whereby As(III) is oxidised to As(V) using H 2 O 2 , lowers the leaching of arsenic, and other contaminants, from the solidified product. With the speciation program MINTEQA2, it is calculated that the solubility of As(V) in the presence of a pure Ca 3 (AsO 4 ) 2 precipitate is lower than the solubility of As(III) in the presence of a pure CaHAsO 3 precipitate. The arsenic concentration in the presence of both a Ca(OH) 2 and a Ca 3 (AsO 4 ) 2 precipitate can even be lowered to 0.47 mg/l (pH 12.5). The As concentration in the leachate of the extraction test on an oxidised S/S sample was indeed lowered to ca. 0.5 mg/l, which is a reduction by a factor of 10 compared to the concentration of ca. 5 mg/l, obtained in the leachate of the extraction test on a non-oxidised S/S sample. This is in very good agreement with the calculated value of 0.47 mg/l. Also, the pretreatment decreased the cumulative fraction of arsenic released over the entire test period of a semi-dynamic leach test by a factor of 7. At all times during the test, the As concentration did not exceed the norm of 1 mg/l.