Mixtures of high-molecular-weight hydrogenous, syndiotactic poly(methyl methacrylate) with two lower-molecular-weight deutero poly(methyl methacrylate)s have been investigated over a range of temperatures and compositions using small-angle neutron scattering. Effective interaction parameters have been obtained which have been used to estimate the Flory-Huggins interaction parameter. Spinodal temperatures have been defined from the temperature dependence of these effective interaction parameters and other parameters of the mixtures. The conclusion is that these isotopic mixtures are incompatible at ambient temperatures and have an upper critical solution temperature. The influence of local stereochemistry on the polymer configuration is evident in the scattering cross-section variation with scattering vector. This variation is compared with the scattering obtained from Monte Carlo rotational isomeric state calculations.