A sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula RS(=O)₂–OH, where R is an organic alkyl or aryl group and the S(=O)₂–OH group a sulfonyl hydroxide. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. The parent compound (with the organic substituent replaced by hydrogen) is the hypothetical compound sulfurous acid.  Salts or esters of sulfonic acids are called sulfonates.