The gas phase relative acidities of 15 sulfonic acids, RSO3H, have been determine via Cooks' kinetic method. The following acidity scale was established (starting with the weakest acid): methane sulfonic acid < ethane sulfonic acid < methoxy sulfonic acid < sulfuric acid < m-aminobenzene sulfonic acid - p-aminobenzene sulfonic acids < p-toluene sulfonic acid < benzene sulfonic acid < o-aminobenzene sulfonic acid < 5-sulfosalicylic acid < o-nitrobenzene sulfonic acid < cysteic acid < m-aminobenzene sulfonic acid < p-nitrobenzene sulfonic acid < triflic acid. In addition the gas phase fragmentation reactions of the conjugate bases, the sulfonate anions RSO3−, have been studied using MS/MS techniques. A general loss of R, resulting in the formation of the radical anion SO3,−is observed in all cases. Other fragmentation pathways are discussed in detail.