The material studied, a waste fly ash from the metallurgical industry, contains the toxic element As in high concentrations, ranging from 23% to 47% (wt %). Besides As, Sb and Ph are present in the waste material. The waste was solidified with inorganic materials such as cement and pozzolanic materials in order to reduce the leachability of the contaminants from the waste. The optimal solidification recipe lowered the As concentration in the leachate of the extraction test DIN 38 414 S4 from ca. 5 g/L to ca. 5 mg/L. In this paper, it was shown that this enormous reduction in As concentration was due to the formation of CaHAsO3 in the leachate, in the presence of Ca(OH)(2). Formation of CaHAsO3 alone cannot lower the arsenic concentration beneath ca. 55 mg/L, as was calculated with the speciation program MINTEQA2.