The growth of epitaxial metal-oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2 x 2) symmetry through the first two monolayers of growth which is followed by a (root 3 x root 3)R30 degrees phase that is attributed to the growth of a Cr2O3(0001) overlayer. Ultraviolet photoelectron spectroscopy measurements have been performed on the CrOx/Pt(111) system. The electronic structures of CrO2, Cr2O3, and Cr3O4 were calculated using the linear muffin-tin orbital method in the atomic sphere approximation. Comparison of the photoemission valence band spectra with the calculated density of states indicates that the CrOx initially grows in a cubic spinel Cr3O4 structure. Beyond similar to 0.2 monolayers, the metallic behavior of the CrOx overlayer begins a transformation to an insulating state. The measured valence emission for the p(2 x 2) phase beyond similar to 0.2 monolayers is more consistent with either a gamma-Cr2O3(111) overlayer or possibly a reconstructed Cr2O3(0001) overlayer.