Pure and doped zirconia aerogels have been characterized in order to understand their thermochemistry in oxidative and reductive atmospheres. Thermal analysis (TG–DTA) indicates that the aerogels underwent crystallization at lower temperatures in reductive atmospheres than in oxidative ones. K-doped ZrO₂ crystallized at much higher temperatures than pure and Y-doped ZrO₂ in both atmospheres. Residual gas analysis combined with IR spectroscopy (FTIR) during oxidation-reduction revealed that the surface residuals (i.e. ethanol, acetate and water, etc.) remaining in the samples after supercritical drying were oxidatively decomposed in the oxidative atmosphere and hydrogenatively decomposed in the reductive atmosphere. Both Y- and K-doped ZrO₂ were reactive to hydrogen and oxygen at high temperatures, but probably by different mechanisms. Charged molecular oxygen was present on the surface of K-doped ZrO₂ at temperatures > 673 K. As a result, K-doped ZrO₂ has been shown to be more active towards hydrogen than Y-doped ZrO₂ at relatively low temperature. The importance of this reactivity is considered.