The effects of preparation methods, humidity, and calcination temperatures on the behavior of nanoscale TiO2 photocatalysts were investigated in this study. Application of these photocatalysts in the gas-phase decomposition of l-butene demonstrated that the nanoscale catalysts, prepared by the acid-assisted sol-gel technique, showed higher photocatalytic reactivity than commercially available Degussa P-25 TiO2, while lower photoactivity was obtained on TiO2 catalysts prepared without adding acid to titanium isopropoxide. Our experimental results revealed that the oxidation rates of I-butene decreased exponentially with increasing water concentrations in the flowing stream. However, a trace amount of water vapor was indispensable in maintaining the stability of the catalysts. Proper calcination temperature (300 degrees C) could promote the resistance of catalysts against the poisoning effects of humidity. Higher calcination temperature (400 degrees C) unfavorably lowered the photoactivity due to phase transformations occurring at such a temperature. Amorphous and rutile-typed TiO2 showed less photocatalytic reactivity. XRD patterns and BET measurements indicated that moderate surface areas (from 100 to 160 m(2)/g), appropriate crystallite sizes (5 similar to 6 nm), and crystallinity of anatase were beneficial to the photoactivity of TiO2 catalysts. In situ FTIR studies indicated that catalyst surfaces contained large amounts of chemisorbed mater and hydroxyl groups, which are considered to be active sites in photocatalytic reactions. The accumulation of carbonate species on active sites resulted in the deactivation of TiO2 catalysts under dry conditions. Quantum size effects were thought to be responsible for the high photoactivity achieved on the nanoscale TiO2 catalysts prepared by sol-gel methods.