Small-angle neutron-scattering measurements have been performed on mixtures of poly(ethylene oxide), PEO, with protio- and deuteriopoly(methyl methacrylate), PMMAH and PMMAD, respectively, to evaluate the Flory-Huggins interaction parameter XAB. It has been found that XAB varies from -5 × to-1 × 10-3as the total PMMA concentration increases from 0.3 to 0.7. The temperature dependence of the scattering combined with other results indicates that entropic, rather than enthalpic, contributions dominate XAB. The magnitude of XAB is found to be on the same order of magnitude as that expected between PMMAH and PMMAD, particularly at high total concentrations of PMMA. Studies of PMMA at higher scattering vectors show that down to length scales of 25 A no changes are observed in the conformation of PMMA in the mixtures in comparison to that of PMMA in the bulk. © 1987, American Chemical Society. All rights reserved.