We prepared thermoresponsive and microporous polymer hydrogels by gamma-ray irradiation of aqueous solutions poly(vinyl methyl ether) (PVME) at different heating rates. Under all temperature programs, opaque and heterogeneous PVME gels formed, which swelled at temperatures below the lower critical solution temperature and shrank at temperatures above it. All of the samples contained porous and phase-separated structures. The shape and size of the gel pores varied depending on the temperature programs. Gels having a sponge-like continuous porous structure formed only when the radiation-induced crosslinking was carried out at an optimum heating rate, which we found to be 0.11-0.13degreesC min(-1). For temperature changes between 10degreesC and 40degreesC, gels with this structure showed rapid volume transitions on a time scale of about a minute. (C) 2004 Elsevier Ltd. All rights reserved.