The determination of porous structure of fragile materials from classical methods such as mercury porosimetry and nitrogen adsorption-desorption methods is often delicate and can be source of false structure interpretation. The thermoporometry is an alternative method, sometimes the unique method, allowing a better characterization of porous fragile and soft materials and hydrogels. In this paper we report on the use of thermoporometry as a technique allowing the determination of pore size distribution in mesoporous materials. First, the reliability of the technique was tested on rigid mesoporous silica with known pore size distribution. The expected pore radius distribution curve was obtained. Thereafter, the technique was applied to characterize the mesoporous structure of poly(N-(2-hydroxypropyl) methacrylamide) (PHPMA) gels swollen in aqueous medium. The pore size distributions determined were compared to those obtained from nitrogen physisorption isotherms.