Isotactic poly(propylene) (PP) was grafted with m-isopropenyl-alpha,alpha-dimethylbenzylisocyanate (TMI) using tert-butyl cumyl peroxide as free-radical initiator. The amount of grafted TMI increased with increasing initial initiator concentration, whereas the molecular weight of PP decreased. An increase of the amount of grafted TMI resulted in a decrease of the PP melting point, onset-temperature, and heat of fusion. The TMI-grafted PP was used as compatibilizer in PP/polyamide 6 and PP/polyamide 66 blends.