A process is described for preparing alkyl thiodiglycolates of the general formula (I) R—OOC—CH2-S—CH2-COO—R  (I) where R is a radical of branched or unbranched C1 to C10-alkyl, characterized in that an alkyl haloacetate of the general formula (II) X—CH2-COO—R  (II) where X is a chlorine or bromine atom and R is as defined for compounds of the formula (I) is reacted with an aqueous solution of alkali metal sulphide or alkali metal hydrogensulphide in the presence of an aqueous pH buffer solution in the pH range between 5 and 8, optionally in the presence of a phase transfer catalyst.