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 μm), but also beneath the PE surface (~ 200 μm) 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.