An analysis of a process for recovery of both products of a double decomposition reaction through two consecutive evapocrystallization-washing steps is conducted on the basis of a simple expression for the heterogeneous equilibrium between the precipitated solids and liquor ions concentrations. The expression is based on lumping the system's non-idealities in terms of an interaction parameter. The developed model is applied to synthesis of the process for the production of K2SO4 and NH4Cl by double decomposition. The presented results illustrate the trends among the various critical internal design variables and their sensitivity to the interaction parameter. This enables an optimum trade off between the operating conditions subject to verification through a limited set of pilot tests. (c) 2005 Elsevier B.V. All rights reserved.