Weight loss curves of wood fast pyrolysis oils in air have been measured, under controlled thermal conditions, carrying out two separate sets of experiments. The first, which has a final temperature of 600 K, concerns evaporation/cracking of the oil and secondary char formation, processes associated with sample swelling and solidification. After collection and milling, in the second set of experiments, heterogeneous combustion of the secondary char is carried out to temperatures of 873 K. Although the details of the rate curves appear to be dependent on the commercial process (BTG, Dynamotive, Ensyn, Pyrovac) applied to produce the oil, the same qualitative features are observed in all cases. Secondary char formation and sample modification begin for temperatures of about 460-490 K. Moreover, a conceptual reaction mechanism, consisting of six main zones, can always explain the low-temperature (less than or equal to600 K) devolatilization characteristics. Similar to primary char produced from wood pyrolysis, secondary char exhibits weight loss curves that present a devolatilization stage and a combustion stage, but with a significantly lower reactivity, as a consequence of a physical structure completely lacking micropore networks.