A mean field theoretical model is developed to investigate the porosity of cross-linked copolymer networks formed by polycondensation. It is found that cross-linking induces microphase separation, and when accompanied by polycondensation, the pore size distribution is affected in an unexpected manner. Pores much larger than the size of the leaving groups are predicted when the leaving group is large. The pore distribution in such cases is found to be multimodal, exhibiting a peak for small pores, a second narrow peak for pores corresponding to the monomer size, and a third peak for very large pores. The model also predicts the extent of microphase separation of the chemically different monomers as a function of the chemical constitution, the degree of cross-linking, and the size of the leaving group.