A method is proposed to estimate the enthalpy of bio-oil in the vapor phase, H-bo, as a function of temperature in the range 298.15-1000 K, of which experimental determination has not been done so far. The two equations proposed in this work allow estimation of the standard enthalpy of formation, H-bo,H-0, and the difference in the enthalpy between 298.15 K and a given temperature, Delta H-bo(T), respectively, only based on the overall C, H, and 0 contents of crude bio-oil of that N and S contents are lower than 0.5 and 0.1 wt %-daf, respectively. These equations were optimized using thermodynamic data of 290 and 141 organic compounds for H-bo,H-0, and Delta H-bo(T), respectively. Given the yields of bio-oil, char, and gas, elemental compositions of the bio-oil and char, and chemical composition of the gas, proposed equations predict the heat required for biomass pyrolysis, Q(py)(T-py), which is defined as the enthalpy difference between the products at the pyrolysis temperature, T-py, and the biomass at 298.15 K. The predicted Q(py) at T-py = 773-823 K for five different types of dry biomass was in the range of 1.1-1.6 MJ kg(-1).