Biomass autothermal flash pyrolysis has been carried out by adding oxygen to the reaction environment in a bench-scale plant provided with a conical spouted bed reactor, at 500 C and using two oxygen concentrations in the inlet gas stream (2.5 and 4.1 vol %, which correspond to equivalence ratios of 15 and 25, respectively). An increase in the oxygen concentration leads to an increase in gas and water yields, whereas there is a decrease in the yield of both char (partially burned), and organic compounds in the bio-oil, albeit only slightly in the latter case. Bio-oil composition undergoes only minor changes for certain individual components. The experimental results' highlight the advantages of the conical spouted bed reactor (CSBR), for scaling up the autothermal operation. A study on energy requirements reveals that the oxygen concentration needed to achieve autothermal operation decreases as the scale is larger. The oxygen content required in the inlet gas stream for autothermal operation in an industrial-scale plant of 500 kg h(-1) is 2.7 vol %, and neither bio-oil yield nor composition is significantly affected, being similar to the case of operating in an inert atmosphere.