Abstract: The invention relates to a process for the treatment of a used waste alkali, in which the used waste alkali is oxidized at a superatmospheric pressure in the range from 60 bar to 200 bar. The pressure of the used waste alkali L is increased and then heated by indirect heat exchange. The heated used waste alkali is conveyed into a separator, wherein vaporized aqueous phase is separated from the used waste alkali. The resultant liquid phase is brought to the desired reaction pressure and introduced into an oxidation reactor. In the oxidation reactor, the used waste alkali is oxidized. In a first reaction, thiosulphates are formed from the sulphides. In a second reaction, the thiosulphates are converted into more stable sulphates.