Sulfenic acids (RSOH) are reactive intermediates in the oxidation of protein cysteines. Among cysteine oxoforms, RSOH represent redox-reversible species that can thus participate in regulation and signaling mechanisms and play key roles in enzyme catalysis and antioxidant activity. How the cysteine (CyS) thiol groups of the human surfactant protein that lines the lung epithelium react with inhaled ozone is deemed critical in preserving Structural Integrity and immune functions. Here we report the simultaneous detection, by online thermospray ionization mass spectrometry, of cysteine sulfenate (CySO-) and the overoxidized cysteine sulfinate (CySO2-) and cysteine sulfonate (CySO3-) species on the surface of aqueous CyS microdroplets exposed to O-3(g) for < 1 ms. These species are produced by rapid, sequential O-atom additions whose relative rates are herein quantified for the first time. From the pH-dependence of ozonation rates, we derive pK(a)(CySOH) = 7.6 +/- 0.3 < pK(a)(CyS) = 8.3.