Condensation of pentan-3-one, hexan-2-one, 4-methylpentan-2-one, 5-methylhexan-3-one, or 2,6-dimethylheptan-4-one with formaldehyde and sodium methanethiolate, which is present in sulfidic spent caustic from the Orenburg gas processing plant, yields the corresponding mono- and bis[(methylsulfanyl)methyl]-substituted ketones or their mixtures depending on the reactant ketone structure. Alkylthiomethylation of butan-2-one with equimolar amounts of formaldehyde and sodium methanethiolate results in 3-methyl-5-thiahexan-2-one or, in the case of a twofold excess of the reagents, in 2-methyl-1,5-bis(methylsulfanyl)-4-[(methylsulfanyl)methyl]pentan-3-one. On the basis of 3-methyl-5-thiahexan-2-one, new polyfunctional [(methylsulfanyl)methyl]-substituted alcohol, -hydroxymethylketone, 1,3-diol, and 1,3-dioxane have been obtained.