The morphology of polystyrene/poly(methyl methacrylate) (PS/PMMA) diblock copolymers was investigated as a function of temperature by using small-angle X-ray scattering. It was found that the microphase separation was enhanced up to temperatures in excess of 200 °C. At 175 °C the copolymers remained microphase separated when mixed with PS or PMMA homopolymer at homopolymer concentrations less than 50%, provided the molecular weight of the homopolymer was less than or equal to the total molecular weight of the copolymer. The temperature dependence of the tracer coefficients D⋆HC, of deuteriated polystyrene (d-PS) chains diffusing into two symmetric PS/PMMA diblock copolymers was studied by using forward recoil spectrometry (FRES). D⋆HC/T was found to have the same temperature dependence as D⋆HH/T, where D⋆HHis the tracer diffusion coefficient of d-PS chains diffusing into PS. A comparison of the temperature dependence of the zero-shear rate viscosity, η0, of PS indicates that D⋆HC/T and ηo-1have essentially the same temperature dependence. Experiments on the diffusion of deuteriated poly(methyl methacrylate) (d-PMMA) also indicated that D⋆HC/T and ηo-1 have the same temperature dependence. © 1989, American Chemical Society. All rights reserved.