Sequential interpenetrating polymer networks (SIPNs) composed of poly(2-hydroxyethyl methacrylate) (PHEMA) and polystyrene (PS) were synthesized by photopolymerization at room temperature. A common solvent, N,N-dimethylformamide, was used to impregnate the styrene monomer into the PHEMA network. The IPNs had a microphase-separated structure of the order of 100 nm. It was found that the gradient composition in the surface region (to a depth of 0.1 mm) was obtained by polymerization inhibition with oxygen which had diffused from the surfaces of the swollen PHEMA gel. X-ray photoelectron spectroscopy measurements revealed that the PS component was enriched in the surface of the IPN. Thermodynamic effects on the composition of the outermost surface layer are discussed.