The effect of synthesis and reaction conditions on the structure and activity of An clusters supported on nanocrystalline and mesoporous TiO2 was investigated. X-ray absorption spectroscopy was applied to correlate the particle size and the oxidation state with several parameters, such as pH of the precursor solution, An loading, pretreatment, and support structure. The study using mesoporous TiO2 as the support shows that lower Au loadings resulted in bigger An aggregates with lower reducibility. The high activity state for Au supported in different allotropic forms of TiO2 corresponds to Au in a fully reduced state. Furthermore, once reduced, no reoxidation occurs under reaction conditions, even after flowing air at higher temperatures (150 and 300 degreesC). Therefore, our results indicate that oxidized Au is not necessary for high activity. The activity decreased with particle growth, and, compared to the other allotropic TiO2, An on brookite exhibited no significant particle agglomeration and was the most active catalyst after treatment at 300 degreesC.