To prepare peloids for thermotherapy, the thermal behaviour of their component clay pastes needs to be known. This study was designed to experimentally determine the specific heat capacity and cooling kinetics of pastes prepared by adding different proportions of water to eight commercially available clays of different composition. According to exponential equations fitted to the cooling curves for the pastes, a new parameter designated the relaxation time is proposed. Using this parameter, the clayey pastes could be classified on the basis of rate of heat release. According to its specific heat capacity and relaxation time, Na-activated magnesium bentonite emerged as the most suitable clay material to prepare peloids with applications in thermotherapy.