The effect of ambient humidity on the current-voltage characteristics of Cu-PS (porous silicon) structure was investigated. The humidity-voltaic effect, i.e. the generation of open-circuit voltage (V-oc) in Cu-PS interface in humid atmosphere (up to 300 mV at 95% relative humidity) in dark and day-light illumination is discovered. Humidity-stimulated open-circuit voltage generation is caused by hydrogen component of water-vapor of ambient. The possible mechanism of hydrogen-stimulated voltage generation in Cu-PS interface is suggested. Besides this effect, annealing in the range of 60-200 degreesC in air on V-oc of Cu-PS structures was studied and decrease of values of V-oc depending on duration of annealing was discovered. These changes were attributed to diffusion of oxygen from air and oxidation of copper film. (V-oc-t) data were used for estimation of diffusion coefficients of oxygen in Cu film. The temperature dependence of the oxygen diffusion coefficient in Cu films are described by the relation D = 5.2 x 10(-7) exp(-0.44/kT). The results of this research showed that Cu-PS structures can be perspective for using as hydrogen sensors. (C) 2003 Kluwer Academic Publishers.