The invention relates to a process for preparing 2-oxo-4-methylthiobutyric acid (I), its salts and its derivatives ##STR00001## in which R represents a group chosen from COOH, COOR', NH.sub.2, 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.