The mineralogy, geochemistry and physical and physicochemical properties of three Spanish clays and their mixtures with distilled water were determined. These results were compared with the data obtained by other authors for peloids from spas in Spain and Turkey. The main clay minerals of the samples studied were montmorillonite in Clay1, saponite in Clay2 and kerolite–stevensite in Clay3. The results obtained showed that all three clays share some properties with clays already in use, but Clay1 and Clay2, that are quite rich in smectite, are more suitable when the typical smectite features are needed for therapy. The peloid from saponite was the most suitable for thermotherapy while the kerolite–stevensite sample was not suitable for the preparation of peloids for this use.