An examination is made of the possibility of synthesizing ketosulphides by means of the thiomethylation of ketones by formaldehyde and natural mercaptans contained in gas condensates, crude oils and distillates. Selective extraction with 86% sulphuric acid and with DMSO and fractional distillation produced ketosulphides with a concentration of 60-75%. Ketosulphides of 99-100% concentration were produced by means of thiomethylation of ketones by formaldehyde and mercaptans pre-extracted by alcohol-alkaline extraction or with organic solvents (sulpholane, diethylene glycol).