Semi-interpenetrating polymer networks (semi-IPNs) composed of poly(dimethyl-aminoethyl methacrylate) (PDMAEMA) and poly(ethylene oxide) (PEO) were synthesized by ␥-radiation; three semi-IPNs with 80 : 20, 90 : 10, and 95 : 5 weight ratios of DMAEMA/PEO were obtained by use of this technique. The gel-dose curves showed that the hydrogels were characterized by a structure typical of semi-IPNs and the results of elemental analysis supported this point. The temperature-induced phase transition of semi-IPNs with the composition of 95 : 5 was still retained, with the lower critical solution temperature of PDMAEMA shifting from 40 to 27°C. The temperature sensitivity of the other two semi-IPNs gradually disappeared. The pH sensitivity of three semi-IPNs was still retained but the pH shifted slightly to lower values with increasing PEO content in the semi-IPNs. The effect of PEO content in semiIPNs on their environmental responsiveness was discussed.