The heat of pyrolysis of beech and spruce wood was investigated by means of a differential scanning calorimeter. Wide variations were found for the heat of the primary pyrolysis process, depending on the initial sample weight and on the conditions used in the measurements. However, reporting the heat of the primary pyrolysis process versus the final char yield resulted in a linear correlation. This strong dependency of the heat of wood pyrolysis on the final char yield, that is in turn highly sensitive to the experimental conditions, can explain the uncertainty of the data for the heat of wood pyrolysis reported in the literature. A possible explanation for the variability of the heat of wood pyrolysis depending on the final char yield seems to be an exothermic primary char formation process competing with an endothermic volatile formation process.