The relationship between hydrogen intercalation and deintercalation (coloring and bleaching) in an aqueous tungsten oxide-based electrochromic device is presented. The built-in potential and optical transmission of the device are measured under step-current conditions; both display a hysteresis which can be explained by a two-phase model in which intercalation occurs as the parallel, equipotential filling of the two phases, but where a deintercalation barrier associated with one of the phases imposes an asymmetry. The observed data are well explained by a corresponding equivalent circuit which allows for the prediction of deintercalation behavior from intercalation behavior with a single adjustable parameter, which represents the barrier to deintercalation.