The transient response of a passive iron-chromium alloy is studied by applying an anodic potential step and monitoring the resulting current transient and mass change. The Fe-25Cr model alloy used in this study is sputter deposited on AT cut quartz crystals which are connected to an oscillator circuit. The change in resonance frequency with time resulting from metal dissolution or film growth is monitored and converted into a mass change using the Sauerbrey equation. A formalism is developed for the interpretation of:current and mass transients observed with binary alloys in the passive state. Two selectivity ratios are defined which characterize the reactions at the metal-oxide and oxide-solution interface, respectively. Further quantities of interest are the dimensionless mass/charge ratio determined experimentally and the dissolution ratio which characterizes the relative importance of dissolution and film formation for a given species. It is found that in acid solution a potential step applied to Fe-25Cr in the passive region leads to a mass loss due to preferential dissolution of iron. As a consequence chromium becomes enriched in the passive film. In 0.1 M NaOH, on the other hand, iron does not dissolve readily and the electrode exhibits a mass gain upon application of a positive potential step. XPS analysis of the passive films qualitatively confirms the conclusions derived from EQCM experiments.