Sulfonic acids are strong acids.  They are commonly cited as being around a million times stronger than the corresponding carboxylic acid. For example, p-Toluenesulfonic acid and methanesulfonic acid have pK values of −2.8 and −1.9, respectively, while those of benzoic acid and acetic acid are 4.20 and 4.76, respectively.  However, as a consequence of their strong acidity, their pK values cannot be measured directly, and values commonly quoted should be regarded as indirect estimates with significant uncertainties. For instance, various sources have reported the pK of methanesulfonic acid to be as high as −0.6 or as low as −6.5.  Sulfonic acids are known to react with solid sodium chloride (salt) to form the sodium sulfonate and hydrogen chloride.  This property implies an acidity within two or three orders of magnitude of that of HCl, whose pK was recently accurately determined (pK = −5.9).
Because of their polarity, sulfonic acids tend to be crystalline solids or viscous, high-boiling liquids. They are also usually colourless and nonoxidizing, which makes them suitable for use as acid catalysts in organic reactions.  Their polarity, in conjunction with their high acidity, renders short-chain sulfonic acids water-soluble, while longer-chain ones exhibit detergent-like properties.
The structure of sulfonic acids is illustrated by the prototype, methanesulfonic acid. The sulfonic acid group, RSOOH features a tetrahedral sulfur centre, meaning that sulfur is at the center of four atoms: three oxygens and one carbon. The overall geometry of the sulfur centre is reminiscent of the shape of sulfuric acid.