In thermotherapy, an appropriate and detailed chemical and physical characterization of the different clays and thermal muds is necessary for the use of these products. In order to predict the thermal behaviour of a mud the analysis of its physical properties is required; among these, those that have bigger influence are the specific heat capacity, the thermal conductivity and the density. This work focuses on the study of the behaviour of the specific heat of a bentonitic clay (composed mainly of 55% Na-saturated smectite, 28% sepiolite, and 15% illite) mixed with distilled water or sea water as a function of the water concentration in the temperature range 293.15-317.15K. The experimental data were measured using a Calvet calorimeter, SETARAM BT 2.15, which permits studying with high precision all kind of substances and materials (oils, polymers, hydrates, powders) and simulate almost any process condition (reactions, crystallisation, freezing, etc.). The obtained specific heat values in the studied temperature range are between 851 and 889 J kg(-1) K(-1) for mineral phase, 3479 and 3505 J kg(-1) K(-1) for 19.9% bentonite + distilled water, and 2724 and 2758 J kg(-1) K(-1) for 39.8% bentonite + sea water. The results of this study can be used in the spas or thermal centres to determine the suitability and quality of the bentonitic muds for pelotherapy. (C) 2011 Elsevier B.V. All rights reserved.