The present invention is directed to a pyrolysis method. The method involves providing a biomass and subjecting the biomass, in a reactor operating under conditions of parasitic heat loss of less than 1% of the biomass' chemical energy content, to partial oxidation where, during steady state operation of the reactor, oxygen is provided to the reactor in sufficient quantity to achieve an equivalence ratio of 0.06 to 0.15 to release sufficient energy to support endothermic pyrolysis reactions and produce condensable organic compounds as the major portion of the pyrolysis products.