AbstractA mathematical model is presented for the calculation of the concentrations or activities of complex compounds in the thermodynamic equilibrium state. Ten central atoms and ten ligands which form 400 complexes at the most can be included in the model. Complex stability constants, total concentrations of the central atoms and ligands, temperature and pH‐value constitute input quantities. Activity and temperature dependence of the stability constants are covered by respective subprogrammes. The model is tested by the migration rate of iron(II) within a sand‐filled column with a cation‐exchange capacity of 1 mval/100 g solid matter: If there is an excess of sulphate, the iron(II)‐breakthrough will be considerably earlier, the same holding for the EDTA‐complex, whereas by means of EDTA the iron(II) bonded to the soil by ion exchange can be redissolved. In the batch experiment also the rate of oxidation of iron(II) is dependent on the degree of complexing. The model can be extended for dissolving, precipitation and redox