We have demonstrated that oxidation states in uranium-oxides such as UO2 and U3O8 can be controlled through epitaxial stabilization by growing single-crystal-like films. We have further shown that the growth of polymorphic uranium oxides is feasible by using different substrates with appropriate in-plane lattice parameters. The clearly improved stability of epitaxial uranium-oxides illustrates the significance of crystal lattice pinning on the control of surface chemistry of materials.