In this paper the thermal behaviour of pure arsenic oxides (As2O5.aq and As2O3) and the influence of the presence of reducing agents (glucose or activated carbon) on the thermal behaviour of the arsenic oxides are studied through thermogravimetric (TG) analysis. The TG experiments with pure As2O5.aq reveal that the reduction reaction As2O5 --> AS(2)O(3) + O-2 does not take place at temperatures lower than 500degreesC. At higher temperatures decomposition is observed. Pure As2O3, however, is already released at temperatures as low as 200degreesC. This release is driven by temperature dependent vapour pressures. Comparing these results with earlier observations concerning the thermal behaviour of chromated copper arsenate (CCA) treated wood, suggests that wood, char and pyrolysis vapours form a reducing environment that influences the thermal behaviour of arsenic oxides. Therefore, the influence of the presence of reducing agents on the thermal behaviour of As2O5.aq is studied. First, TG experiments are carried out with mixtures of As2O5 and glucose. The TG and DTG curves of the Mixture are not a simple superposition of the curves of the two pure constituents. The interaction between As2O5.aq and glucose results in a faster decomposition of arsenic pentoxide. This effect is more pronounced if the purge gas nitrogen is mixed with oxygen. Second, TG experiments are performed with mixtures of As2O5 and activated carbon. The presence of activated carbon also promotes the volatilisation of arsenic for temperatures higher than 300 degreesC, probably through its reducing action. Extrapolation of the thermal behaviour of these model compounds to the real situation of pyrolysis of CCA treated wood confirms the statement that the reduction of pentavalent arsenic to trivalent arsenic is favoured by the reducing environment, created by the presence of wood, char and pyrolysis vapours.