Photocatalytic degradation rates of 2-propanol to acetone were examined by using TiO2 films modified with Pd stripes whose interval was varied from 25 mum to 5 mm, to characterize their photoelectrochemical behavior in the gas phase. Even though the ratio of Pd area to bare TiO2 area was kept constant (1:1), the photocatalytic activity increased as the stripe interval was decreased, in the case where a certain amount of NaCl had been deposited on the Pd-modified TiO2. This is indicative of the formation of photoelectrochemical micro-cells in the vicinity of the Pd deposited on the TiO2 thin film in the gas phase. The width of the micro-cell was, for example, estimated to be less than or equal to 250 mum when 0.2,mug cm(-2) of NaCl had been deposited on the sample film. The micro-cell formation required adsorbed water on the TiO2 surface, which should be essential for the dissociation of NaCl; the adsorbed water layer in which NaCl is dissolved should act as the electrolyte of the micro-cell. The width of the micro-cell varied from the order of mum to mm, depending strongly on the amount of NaCl (0.02-20 mug cm(-2)). On the other hand, the width of the micro-cell at the salt-free TiO2 surface was found to be less than 25,um. Comparison between the experimentally observed dependence and the theoretically simulated one suggests that ionic conductivity in the adsorbed water layer is suppressed, probably due to electrostatic and/or chemical interaction between the ions and the TiO2 surface.