The chemistry of chromium in the passive film on sputter-deposited chromium and AlCr thin films has been investigated in situ in an electrochemical cell under potential control by studying x-ray absorption near edge structure. At high potentials, Cr in the AlCr alloys was oxidized to the 6-valent state. Depending on the rate of potential increase, 6-valent chromium either dissolved from the alloy or was trapped in the passive film where it was electroactive, i.e., the valence state could be reversibly switched between the 3- and 6-valent states by changing the applied potential. The kinetics of these processes were investigated. Ex situ x-ray photoelectron spectroscopy measurements indicated that, during slow scanning at low potentials, the composition of both the surface oxide and underlying metallic layers changed. These changes resulted in a structure that was susceptible to transpassive dissolution of Cr at potentials above 0.2 V(MSE).