Differential thermal analysis (DTA) and gravimetric thermal analysis (GTA) were evaluated as techniques for determining the activation energy (E) of the pyrolysis of kraft black liquor. Although more time consuming, DTA gave more meaningful values that were independent of the heating rate and of the (usually unknown) order of the reaction. The use of GTA required assumptions concerning order of reaction, but might be useful for rapid screening. The results may be significant for recovery furnace operation, because the higher Ea, the more difficult it is to maintain combustion. Some typical values of E, in cal/mol, (obtained under a nitrogen atmosphere) for the first exothermic reaction (temperature, °C, in brackets) are: kraft black liquor, 19,400 (290–350); glucose, 9,610 (320–450); kraft liquor plus 10% glucose, 11,100 (240–330); kraft lignin, 7,860 (345–420); spruce wood, 38,100 (290–310).