The invention relates to a process for preparing 2-oxo-4-methylthiobutyric acid (I), its salts and its derivatives in which R represents a group chosen from COOH, COOR′, NH2, NHR′ or NR′R″, where R′ and R″ are chosen, independently of one another, from the group of linear or branched alkyl radicals having from 1 to 12 carbon atoms and cycloalkyl radicals having from 3 to 12 carbon atoms, according to which process but-3-ene-1,2-diol (II) is catalytically and selectively oxidized to give 2-oxobut-3-enoic acid (III) and methyl mercaptan is selectively condensed with 2-oxobut-3-enoic acid (III). 2-Oxo-4-methylthiobutyric acid (I), its salts and its derivatives are used as food supplement, in particular in animal nutrition.