Abstract: An improved kraft pulping process is characterized by the addition of a spent concentrated sulphuric acid composition containing organic matter to a kraft recovery system to provide a mixture enriched in its total sulphur content that is subjected to dehydration, pyrolysis and reduction in a recovery furnace. The organic matter of the sulphuric acid composition is particularly beneficial as a source of thermal energy that enables high heat levels to be easily maintained to facilitate the oxidation and reduction reactions that take place in the furnace, thus resulting in the formation of sulphide used for the preparation of cooking liquor suitable for pulping.