The effect of polymers, in particular dextran,on the equilibrium of bicontinuous microemulsion, oil, and water phases was studied on the H2O-C12E5-C10H22 system. Whereas no dextran was incorporated into the upper (u) oil phase, its partitioning among the microemulsion middle (m) and water lower (l) phases was strongly affected by the molecular weight of the polymer. At low molecular weight, the distribution was approximately uniform, whereas at high molecular weight almost all the polymer was concentrated in the l-phase. The molecular weight cutoff (in terms of the end-to-end distance of the dextran macromolecular coil) was comparable with the size of water domains in the bicontinuous microemulsion structure. Raising the dextran content caused the lower and upper phases to grow at the expense of the middle phase. This pattern of volume effects was shown to be characteristic of some other water-soluble (poly(ethylene glycol)) and oil-soluble (poly(isobutylene)) polymers. The fact that the high-molecular-weight dextran was not incorporated into the bicontinuous microemulsion phase enabled us to analyze the volume changes induced by the polymer in terms of the osmotic pressure effects, by analogy with the osmotic stress experiments pioneered by Parsegian et al. By using the Gibbs-Duhem equation, the chemical potentials of the microemulsion components were evaluated at a series of concentrations and then fitted with the microemulsion theory. From this fit, the C12E5 monolayer bending modulus was estimated.