Variations of compositions of Montecatini Terme waters during anannual cycle are studied. The waters are originated by a mixture of a deeper component, which flows through evaporitic layers and a superior one, fed by a superficial circulation. The composition of waters is constant for every constituent except $SO^{2-}_{4}$; this variation is attributed to different leakage grade of evaporitic layers; a metamorphic process with ionic exchange between Ca of clays and Na of waters and viceversa is taken into consideration. The variations in the water supply of the springs is attributed to fluctuations of the deeper supply. The source basin of both hydrologic components is essentially provided by the pervious formations of the zone.