Theoretical calculations based on statistical thermodynamics are presented addressing the partitioning behavior of a monomer, solvent, or plasticizer in a two-phased polymer system. A phase-separated interpenetrating polymer network (IPN) consisting of a polyurethane (PU) and poly(methyl methacrylate) (PMMA) (50/50 by weight) is chosen as the model. Methyl methacrylate (MMA) is chosen as the partitioning small molecule, which is added at 20% by weight of the polymers. It is shown that the free energy of mixing this MMA is minimum only when the MMA is partitioned nearly uniformly between the two phases. It is also shown that such mixing of a small molecule in a polymer system is controlled by the entropic rather than enthalpic contributions to the free energy changes.