Mixtures of linear poly(vinyl methyl ether) (PVME) with four-armed star polystyrene (PS) were studied to evaluate the effect of chain topology on the critical fluctuations in homogeneous polymer mixtures. It was found that the cloud point curve of the PS/PVME mixtures were elevated by ~10 °C over that of the corresponding linear mixtures. Use of deuterated PS elevated the cloud point by another ~10 °C. Small-angle neutron scattering SANS on mixtures of the deuterated PS with PS was used to evaluate the single chain structure factor of the PS. The agreement between the theories of Benoit and Burchard for branched molecules and the experimental structure factor was excellent. Using this structure factor the concentration fluctuations in the homogeneous mixtures was investigated by SANS as a function of concentration and temperature. The inverse of the intensity at zero scattering vector, S(0)-1 extrapolated linearly with the inverse temperature to yield the spinodal temperature Ts. The correlation length, ξ, was found to depend upon [(T - Ts)]/Ts]-ν where ν = 0.5. ξ-2 was also linearly dependent on T-1 yielding a value of Ts at ξ-2 = 0 in close agreement with the S(0)-1 results. ξ0, the bare correlation length, was found to depend upon composition in a manner similar to a previous study. Finally, the Flory-Hug-gins interaction parameter was found to vary with 1/T and to depend upon composition. © 1990, American Chemical Society. All rights reserved.