The factors controlling leachate composition of cement stabilized air pollution control (APC) residues (41% APC residues, 22% cement, 3% Na2CO3, and 32% water, w/w) have been investigated both in the laboratory ansi in a pilot landfill. Batch leaching and tank leaching tests were carried out in the laboratory in order to determine solubility controlling phases and diffusion controlled species. The major species Ca, SO4, Al, and Si could be partially modeled by assuming calcium silicate hydrate (C-S-H), portlandite, and ettringite to be the solubility controlling phases both in field and laboratory. There were obviously additional minerals that could not be taken into account in calculations because of the lack of data. The determined effective diffusion coefficients (D-e) for Na and K (2.18e(-12) and 5.43e(-12) m(2)s(-1)) were used to model field concentrations. Agreement with field data was good. Heavy metal concentrations were in the range of 10(-8) mol dm(-1) (Cd, Co, Cu, Mn, Ni) to 10(-6) mol dm(-1) (Mo, Pb, W, Zn) in all experiments and often lower in the field leachate than expected from batch experiments. In laboratory experiments, the solubility of Mo and W was most probably controlled by their calcium metalates, Cu by CuO, Ni by Wi(OH)(2), and Zn probably by a Zn containing C-S-H phase. In the field, diffusion seems to control Mo and W leachability, with calculated D,values of 3.49e(-14) and 1.35e(-15) m(2) s(-1).