With the electrochemical quartz crystal microbalance, it is possible to obtain time-resolved in situ estimates of thickness changes of passive films. Thickness changes recorded for physical vapor deposited pure Cr during potential scans in the passive region in acidic media were compared to two different theories for passive film growth: the potential change is governing the growth rare through a reaction at either of the oxide film interfaces (interface model), or it controls the ion conduction current through the film (high field model). The two models proved equally successful in predicting the film growth curves for varying potential-step amplitudes and sweep rates. However, for experiments with different initial passive film thicknesses, the interface model was more successful in predicting film growth.