Hydrogen transport through electro-deposited nickel hydroxide films containing various fractions of cobalt hydroxide has been investigated by analysing build-up and decay current transients under the application of various voltage steps from 70 to 400 mV. The build-up and decay transients of hydrogen flux into and from the electrode film with reversible trap under the impermeable boundary condition were numerically simulated on the basis of the McNabb and Foster's physical model of hydrogen trapping. The simulated transients were compared to the measured data. The occurrence of current plateau upon hydrogen extraction from the completely hydrogen-injected films indicated that the hydrogen transport through the film proceeds by the movement of NiOOH/Ni(OH)(2) phase boundary. The velocity and mobility of the phase boundary movement upon hydrogen extraction were determined to be orders of 10(-6) cm s(-l) and 10(-5) cm s(-1) V-1 in magnitude, respectively. It is concluded that both the velocity and mobility of the NiOOH/Ni(OH)(2) phase boundary movement is raised by the Co(OH)(2) incorporation into the NI(OH)(2) film. From the appearance of build-up and decay current transient curves, it is suggested that Co(OH)(2) incorporation into the Ni(OH)(2) film enhances the release of the trapped hydrogen. (C) 1997 Elsevier Science Ltd.