Abstract Apparent shear and dynamic complex viscosity measurements at 200°C and 230°C, for a series of blends of a homopolypropylene (PPVC) with maleic anhydride polypropylene (PP‐g‐MA) and blends of an ethylene‐propylene copolymer (PPSC) with PP‐g‐MA, are reported. The data were taken using a capillary rheometer and a dynamic mechanical thermal analyzer. The results are analyzed in terms of the Cox‐Merz rule. Data taken at 230°C for both matrices (viz. PPVC and PPSC) follow the Cox‐Merz rule. However, deviations are noted in the blends, especially in compositions wherein PP‐g‐MA content is above 10 wt%. Cross phase separation and charges in the dispersed phase size (of PP‐g‐MA) in the two types of measurement appear to be the cause of the failure of the Cox‐Merz rule.