The liquid phase coking of waste kraft pulping liquors can be effected with a shorter residence time in the coking means at lower temperature, and lower pressure by reducing the pH of such liquors by at least one pH unit before coking. Products are lower in malodorous organic sulfur components than those produced at a higher pH.The lowering of the pH is achieved by adding sulfur dioxide to the liquor either as free SO.sub.2 or as an aqueous solution of SO.sub.2.