Method for treatment of spent liquor at a pulp mill, in which method at least a part of the spent liquor flow arriving from the evaporation plant is taken to a pyrolysis reactor, wherein it is pyrolysed at a temperature of 300-800.degree. C. in order to separate evaporable compounds from the coke remaining in a solid state. The pyrolysis products, which are gases or liquids, may be used as fuel or they may be processed further. The coke resulting from the pyrolysis is burnt in a soda recovery boiler or in a gasification reactor to regenerate cooking chemicals. The method is suitable for recovery of chemicals and energy both in sulphate and sulphite processes and also in cooking methods based on organic solvents.