A theoretical model is developed to explore the porosity and chemical morphology of copolymer membranes. The size of the leaving group during polycondensation of the cross-links is found to have unexpected effects on the porosity, leading to a nonuniform pore-size distribution. The size and formation of microdomains with different chemistries and porosities can be predicted using this model. Such knowledge of a membrane's physical and chemical morphology is useful in the development of fouling-resistant filtration membranes. Reverse osmosis and nanofiltration membranes commonly used today frequently undergo degradation due to fouling and thus experience a diminishing water flux. Additional control over the properties of the membrane can be achieved by combining two or more polymers to form a heteropolymer network.