Abstract Immiscible blends of polycarbonate (PC) and polyacetal (polyoxymethylene, POM) were investigated through rheological measurements, tensile, impact, TEM, SEM, FTi.r. and thermal analysis. The large difference in melt viscosities between these two matrix polymers makes this blending system extremely difficult to process and results in partial thermal degradation of POM. The formation of a strong interpenetrating interface in PC/POM blends significantly raises tensile yield strength, modulus and heat distortion temperature above the average values for the individual polymers. Thermoplastic polyurethane elastomer is not very efficient in toughening PC/POM blends because the elastomer distributes selectively in the PC phase. The rheological morphologies of the blends are interpreted through TEM and SEM photomicrographs.