The present study investigates the influence of the operative conditions on the heat of pyrolysis of four energy crops. Focus is mainly on the role of system pressure in defining heat of pyrolysis and the residual char yield. An experimental investigation has been carried out to measure these parameters by the integrated use of thermogravimetric analysis and differential scanning calorimetry. The results evidence that an increase in the operating pressure reduces the heat requirements of the pyrolysis process and the thermal effect of pyrolysis reactions may shift from endothermic to exothermic. These findings can be interpreted considering that the experimental conditions limiting mass transfer provide a higher residence time of the volatiles products from primary thermal degradation reactions. These primary volatiles react with the pyrolysis products, resulting in exothermic reactions with the formation of secondary char. The heterogeneous secondary reactions lead to carbon enrichment of the final residue and reduce the overall thermal effect of the conversion. Moreover, the heat of reaction as a function of pressure was shown to fit a Langmuir adsorption curve. The results suggest that the role of exothermic secondary reactions and the inhibition of the evaporation of high molecular weight compounds formed in the primary pyrolysis process may be among the main factors affecting the heat demand of the overall pyrolysis process.