The pore size distributions, PSDs, of hydrogel-hollow-fiber membranes such as poly(methyl methacrylate), cellulose triacetate, polyacrylonitrile and polysulfone utilized as artificial kidneys have been characterized using differential scanning calorimetry via thermal porosimetry, termed thermoporosimetry by the authors. The nitrogen gas adsorption-desorption technique was applied to the membranes after freeze-drying to determine the PSDs and, moreover, thermoporosimetry was applied to the membranes before freeze-drying and those refilled with water after freeze-drying, i.e. after nitrogen gas adsorption-desorption measurements. As a result, for the freeze-dried membranes, the PSDs determined using nitrogen gas desorption were found to be generally in good agreement with those obtained using thermoporosimetry. In addition, the PSDs of the poly(methyl methacrylate) membrane before and after freeze-drying were found to be quite different, although the PSDs of the cellulose triacetate, polyacrylonitrile and polysulfone membranes were nearly similar before and after freeze-drying.