Therapeutic muds are used in the treatment of illnesses of the locomotor apparatus, including osteoarthritis and rheumatologic diseases. The mechanisms of action of this therapy are a matter of discussion, mainly for the different traditions of pelotherapy centers. Heat plays a fundamental role in the beneficial effects of thermal mud therapy together with the possible transfer across the skin barrier of chemical elements presented in the mud. Preparation procedures of therapeutic muds have been orally transmitted since ancient times, being accepted that muds require a “maturation” process to achieve the desired therapeutic results. Pharmaceutical research of maturation is crucial to ascertain the possible changes induced by this operation in the properties of muds. In particular, it is necessary to verify the changes associated with physical and/or chemical therapeutic mechanisms that sustain the traditional use of maturation in the preparation of therapeutic muds. Two clay samples were used to prepare thermal muds with mineral medicinal water from the thermal spring of Graena (Cortes y Graena, Granada, Spain). Muds were matured for three months and characterized over time for those properties considered relevant in view of their topical administration (rheological properties and particle size distribution) and possible mechanisms of action (composition, pH, cation exchange capacity, thermal properties and amount of cations released). Maturation of the studied therapeutic muds did not induce alteration of clay minerals, even if a decrease in amplitude of particle size distribution, changes in pH and disappearance of thixotropic behavior were observed. Maturation increased the release of cations from therapeutic muds but did not improve their thermal properties. In the studied case, thermophysical activity did not require maturation. Conversely, maturation increased the amount of cations released from the muds, appearing as a beneficial process for possible chemical therapeutic effects associated with the ionic content of these systems. Maturation could therefore explain the differential chemical effects associated with the use of therapeutic muds compared to other thermotherapeutic agents.