A method of preparing digesting liquor having high sulphidity from a spent liquor obtained from digesting cellulosic fiber material, preferably without a conventional causticizing step is described, said method comprising thermal decomposition of the spent liquor under reducing conditions in a reactor at a pressure of from atmospheric pressure up to about 150 bar and at a temperature of about 500 to 1600 °C so that a combustible gas phase containing hydrogen sulphide is formed in the reactor and extracted therefrom, and a phase of solid or molten material comprising substantially sodium sulphide which is dissolved in aqueous liquid to produce said digesting liquor. According to the invention hydrogen sulphide is recovered from the gas phase and is returned to the reactor to be present during the thermal decomposition of the spent liquor.