In recent years, industrial interest in condensation copolymers with controlled microstructures has been increasing as these systems add an additional dimension to the design and manipulation of product properties without requiring completely new routes for monomer or polymer synthesis. The techniques used to control the compositional microstructure in condensation systems differ greatly from those in vinyl polymerization, as condensation polymers are continuously broken apart and reformed during the course of the polymerization. Blocky copolymers may be produced in a melt blending process only by Limiting the contact time at reaction temperatures because the ultimate result of the polymerization and interchange reactions is complete randomization of the copolymer with a structure similar to that obtained in vinyl polymerization with all reactivity ratios equal to one. The design of processes yielding the desired product microstructure therefore requires a quantitative understanding of the effect of each reaction on the copolymer composition. As typical copolymer recipes include multiple monomers with different functionalities, in this paper a general copolycondensation model is presented that can accommodate an arbitrary number of monomers of differing reactivities. In this paper, only monofunctional and bifunctional monomers are considered; the extension to the case of gelating systems is left for a future paper. The use of this framework and the validity of the approach is demonstrated for an example situation in which a polyarylate is melt blended with PET to produce a copolymer whose average sequence length may be controlled by limiting the extent of reaction.