An investigation has been made of the thiomethylation of ketones by formaldehyde with mercaptides, sodium sulphide and their mixture. It is possible to regenerate 78-100 rel.% of the sulphide-alkaline solutions under mild conditions (2-50-degrees-C, atmospheric pressure) without feeding a catalyst, with the simultaneous production of ketosulphide concentrate -a less toxic product with properties of practical benefit.