The effect of an external electron donor, namely, phenyltriethoxysilane (PES) on the compositional heterogeneity and the microstructure of ethylene-propylene copolymers prepared using a titanium-magnesium catalyst has been investigated. Addition of PES causes an increase in the average sequence length of ethylene and propylene. The products of reactivity ratios of ethylene and propylene indicate that the copolymer is blocky in nature. The external donor produces a propylene sequence in the copolymer which is richer in mmmm pentads.