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The influence of humidity on the decomposition of gas phase chlorobenzene (CB) was studied in a batch photocatalytic reactor using TiO2 (anatase). Increasing relative humidity (>7% at 25°C and 1atm) resulted in an increasingly detrimental effect on the adsorption and photocatalytic decomposition rate of CB. Mechanistic analysis indicates that the relative hydrophilic/hydrophobic nature of a given chemical should dictate adsorption onto or penetration through water to the TiO2 surface. It is suggested that multiple layers of water molecules are formed at the TiO2 interface with air, even at low relative humidities. The multiple-layered film of water retards/prevents CB from reaching the reactive TiO2 surface or contacting radical species in the boundary layer. [Copyright &y& Elsevier]