The aim of the study was to explore the antiradical activity of sulfurous water, used for inhalatory therapy (characterized by the presence of sulfhydryl [HS]) by means of electron paramagnetic resonance (EPR) spectroscopy. The effects of sulfurous water corresponding to the concentrations from 16 down to 0.25 μg/mL of HS were tested by means of Fenton reaction (HO•), KO2-crown ether system (O2-•), and EPR of Tempol and of Fremy's salt radical. All of these assays were made using natural sulfurous water or degassed sulfurous water (no detectable HS) or reconstituted sulfurous water (degassed plus NaHS). The free radicals were significantly inhibited by natural water with HS concentrations ranging from 16 to 1 μg/mL for HO•, Tempol, and Fremy's salt, and O2-• was significantly inhibited from 16 and 2 μg/mL. The tests of degassed water did not reveal any significant differences from baseline values. The tests of reconstituted water led to significant results overlapping those obtained using natural water, thus confirming the importance of the presence of HS group (reductive activity). The positive effects of the activity of sulfurous thermal water is partially based on the patients' subjective sense of well-being and partially on symptomatic (or general) clinical improvements that are sometimes difficult to quantify. These findings indicate that, in addition to their known mucolytic activity and trophic effects on respiratory mucosa, the HS groups in sulfurous water also have antioxidant activity that contributes to the water's therapeutic effects on upper and lower airway inflammatory diseases.