Process for preparing α-keto acids and derivatives thereof of the general formula or in which R 1 is a branched or straight-chain C 1 -C 18 -alkyl, C 5 -C 8 -cycloalkyl, vinyl, allyl, C 6 -C 10 -aryl-C 1 -C 4 -alkylene or C 4 -C 9 -heteroaryl-C 1 -C 4 -alkylene group, where R 1 is optionally substituted, R 2 is -OR''' or -NR'R'', in which R' and R" are the same or different and are each a hydrogen atom or a branched or straight-chain C 1 -C 6 -alkyl group, and R''' is a hydrogen atom, a branched or straight-chain C 1 -C 8 -alkyl, C 5 -C 8 -cycloalkyl, allyl or a benzyl group, where alkyl in R', R'' and/or R"' is optionally substituted, or R"' is an alkaline metal ion, alkaline earth metal ion or a mono- or divalent transition metal ion, R 3 is the same or different and is a hydrogen atom, a branched or straight-chain C 1 -C 8 -alkyl, allyl or a benzyl group, where alkyl and/or benzyl is optionally substituted, or the two R 3 radicals together are C 2 -C 8 -alkanediyl and together form a ring, or the two R 3 and R"' radicals together are part of a C 3 -C 8 -alkanetriyl group of the general formula -R 3 (CH-)R'" and together form a bicyclic group, wherein a) an aldehyde R 1 CHO is reacted with identical or different thiols of the formula R 4 -S-H, in which R 4 is a branched or straight-chain, optionally substituted C 1 -C 6 -alkyl, C 5 -C 8 -cycloalkyl, allyl or benzyl group to give the corresponding dithioacetal, b) the dithioacetal formed reacts with a carbonyl-containing electrophile in the presence of a strong base and after hydrolysis to give the α,α-(dithio)carboxylic acid or derivatives thereof of the formula in which R 5 is a hydrogen atom or a branched or straight-chain C 1 -C 6 -alkyl group and the R 4 and R 1 radicals are each as defined above, and c) the α,α- (dithi o)carboxylic acid or derivatives thereof of the formula are converted by acid-catalyzed solvolysis in the presence of at least 1 molar equivalent of water with release of thiols of the formula R 4 SH to give the α-keto acid or derivatives thereof of the general formula or. R 1 is a branched or straight-chain C 1 -C 18 -alkyl, C 5 -C 8 -cycloalkyl, vinyl, allyl, C 6 -C 10 -aryl-C 1 -C 4 -alkylene or C 4 -C 9 -heteroaryl-C 1 -C 4 -alkylene group, where R 1 is optionally substituted, R 2 is -OR''' or -NR'R'', in which R' and R" are the same or different and are each a hydrogen atom or a branched or straight-chain C 1 -C 6 -alkyl group, and R''' is a hydrogen atom, a branched or straight-chain C 1 -C 8 -alkyl, C 5 -C 8 -cycloalkyl, allyl or a benzyl group, where alkyl in R', R'' and/or R"' is optionally substituted, or R"' is an alkaline metal ion, alkaline earth metal ion or a mono- or divalent transition metal ion, R 3 is the same or different and is a hydrogen atom, a branched or straight-chain C 1 -C 8 -alkyl, allyl or a benzyl group, where alkyl and/or benzyl is optionally substituted, or the two R 3 radicals together are C 2 -C 8 -alkanediyl and together form a ring, or the two R 3 and R"' radicals together are part of a C 3 -C 8 -alkanetriyl group of the general formula -R 3 (CH-)R'" and together form a bicyclic group, wherein a) an aldehyde R 1 CHO is reacted with identical or different thiols of the formula R 4 -S-H, in which R 4 is a branched or straight-chain, optionally substituted C 1 -C 6 -alkyl, C 5 -C 8 -cycloalkyl, allyl or benzyl group to give the corresponding dithioacetal, b) the dithioacetal formed reacts with a carbonyl-containing electrophile in the presence of a strong base and after hydrolysis to give the α,α-(dithio)carboxylic acid or derivatives thereof of the formula in which R 5 is a hydrogen atom or a branched or straight-chain C 1 -C 6 -alkyl group and the R 4 and R 1 radicals are each as defined above, and c) the α,α- (dithi o)carboxylic acid or derivatives thereof of the formula are converted by acid-catalyzed solvolysis in the presence of at least 1 molar equivalent of water with release of thiols of the formula R 4 SH to give the α-keto acid or derivatives thereof of the general formula or. a) an aldehyde R 1 CHO is reacted with identical or different thiols of the formula R 4 -S-H, in which R 4 is a branched or straight-chain, optionally substituted C 1 -C 6 -alkyl, C 5 -C 8 -cycloalkyl, allyl or benzyl group to give the corresponding dithioacetal, b) the dithioacetal formed reacts with a carbonyl-containing electrophile in the presence of a strong base and after hydrolysis to give the α,α-(dithio)carboxylic acid or derivatives thereof of the formula in which R 5 is a hydrogen atom or a branched or straight-chain C 1 -C 6 -alkyl group and the R 4 and R 1 radicals are each as defined above, and c) the α,α- (dithi o)carboxylic acid or derivatives thereof of the formula are converted by acid-catalyzed solvolysis in the presence of at least 1 molar equivalent of water with release of thiols of the formula R 4 SH to give the α-keto acid or derivatives thereof of the general formula or.