A model reactor system was assembled to study surface grafting reactions which would occur at the polymer interface in in-situ blend compatibilization using a vector fluid. The vector fluid's purpose is to convey a reactive ingredient to a blend interface and induce copolymer formation. Polyethylene (PE) was chosen as the polymer substrate, styrene monomer and/or dimethyl phthalate (DMP) as the vector fluid, and a peroxide initiator as the reactive ingredient. The free radical surface grafting reaction of styrene onto the PE surface was studied at melt processing temperature with a factorial experimental design involving the factors of time, temperature, initiator type and initiator concentration. It was found that styrene monomer was grafted at the PE substrate surface, forming a layer of PE-g-PS graft copolymer which was observed with attenuated total reflection Fourier transform infrared spectroscopy. The results indicated that the grafting reactions occurred not only at the immediate surface (2-3 mm), but also beneath the PE surface (ร 200 mm) due to the swelling of the PE by the styrene monomer. The reaction below the immediate surface could be significantly reduced by the presence of DMP, a nonsolvent of PE; but the surface reaction was not affected. Explanations for the reaction behavior of the two different vector fluids are proposed based on the experimental results.