In inorganic chemistry, phosphonic aci is a phosphorus oxoacid with a formula of H3PO3, more commonly known as phosphorous acid. It exists in solution as two tautomers, the major one being HP(O)(OH)2 and the minor one P(OH)3. The former is sometimes termed phosphonic acid, with the latter designated as phosphorous acid. Sometimes confusingly, both these names are also used to refer to H3PO3 in general, i.e. both tautomers.