Weight losses in air of four biomass fast pyrolysis oils (BTG, Dynamotive, Ensyn, Pyrovac), measured under controlled thermal conditions (heating rates of 5, 10, and 20 K/min up to 600 K), are used to formulate a multistep mechanism. This is based on the introduction of eight lumped classes of volatile products and a corresponding number of parallel reactions. The reaction rates are linear in the mass fractions of the lumped product groups and present the usual Arrhenius dependence on temperature. The simultaneous evaluation of all the measured curves produces the same set of activation energies (46, 65, 71, 80, 72, 84, 102, and 71 kJ/mol) and preexponential factors almost invariant with the bio-oil samples. In this way, the differences in the reactivity, deriving from the biomass properties and the pyrolysis characteristics, can be taken into account essentially by the stoichiometric coefficients. A relation is also provided between the eight lumped classes of products and the volatiles generated from evaporation and thermal cracking of bio-oil (permanent gases and 75 liquid compounds previously quantified).