The rheological behaviour of miscible blends of poly(methyl methacrylate) (PMMA) with poly(styrene-co-acrylonitrile) (PSAN) and with poly(vinylidene fluoride) (PVDF), in the molten state, was investigated. For this study, both steady and oscillatory shear flow measurements were made for PMMA/PVDF and PMMA/PSAN blends, which were prepared by melt blending in a twin-screw compounding machine. The following observations were made from the experimental results obtained : (1) plots of the logarithm of the zero-shear viscosity (log eta(ob)) versus blend composition show positive deviations from linearity for the PMMA/PSAN blend system, but negative deviations from linearity for the corresponding PMMA/PVDF system, under isothermal conditions; (2) plots of log eta(ob) versus blend composition for the PMMA/PVDF blend system show minimum values at certain blend compositions as the viscosity ratio of the constituent components becomes smaller than a certain critical value and; (3) logarithmic plots of the dynamic storage modulus (G’) versus the dynamic loss modulus (G") for all of the blend compositions lie between those obtained for the constituent components in both of the PMMA/PSAN and PMMA/PVDF blend systems. The experimental results are interpreted by using a molecular viscoelasticity theory recently developed by Han and Kim.