Thiocarboxylic acids are organosulfur compounds related to carboxylic acids by replacement of one oxygen centre by sulfur. There are two possible tautomers, RC(S)OH and RC(O)SH, which are sometimes written as 'carbothioic O-acids' and 'carbothioic S-acids' respectively. The carbothioic S-acid tautomer RC(O)SH is far more common.