Solid-liquid equilibria in complex aqueous systems involving a heavy metal cation (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, or Zn2+) and one or more ions for which Extended UNIQUAC parameters have been published previously are modeled using the Extended UNIQUAC model. Model parameters are determined on the basis of a data bank with more than 4, 000 experimental data points for binary and ternary aqueous systems. The parameters are generally valid in the temperature range from the freezing point to the boiling point of the respective solutions.