Oxidized white liquor is heat treated to increase the concentration of polysulphide measured at 285 or 286 nm (PS.sub.UV) or measured at 416 nm (PS.sub.VIS) and the PS.sub.UV/PS.sub.GR or PS.sub.VIS/PS.sub.GR ratio, whereby the content of active polysulphide in the total polysulphide is increased which active polysulphide can be exploited to increase pulp yield in Kraft pulping.