This study deals with a chemical survey (major ions and Li+, F−, Br−, and ) on water samples collected from April 1998 to April 1999, from the Telese thermal spring, southern Italy, in order to investigate possible variations in the chemical composition of the spring. Water samples are of the calcium bicarbonate type, with small fluctuations in outlet temperature (20.05 ± 0.23°C), pH (6.23 ± 0.16), and TDS (2415 ± 35 mg/l). The concentrations of major ions stayed nearly constant, except for . All analyzed waters are saturated or oversaturated relative to calcite and dolomite. The higher saturation indices of the water samples relative to calcite and dolomite observed during the summer of 1998 is interpreted in terms of the lower rainfall during the dry season, inducing a lower supply of CO2 to the feeding aquifer at depth. The sharp decrease of in the December 1998 to January 1999 water samples during the wet season may be ascribed to a drop in the supply of H2S from a deepseated source due to a temporary change of stress regime. We suggest that the crustal permeability has diminished in coincidence with the end of a seismic swarm, inducing a change in the sulfate content.