The growth Of MoO3 films on SrLaAlO4(0 0 1), a substrate lattice-matched to beta-MoO3, by oxygen plasma assisted molecular beam epitaxy was characterized using reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy, Xray diffraction (XRD), and atomic force and scanning tunneling microscopies (AFM and STM). It was found that the flux of reactive oxygen species to the surface was not high enough to maintain the proper stoichiometry, even at the lowest measurable deposition rates. Therefore, the films were grown by depositing Mo in small increments and then allowing the Mo to oxidize. At 675 K, the films grew epitaxially but in a three-dimensional manner. XRD of films grown under these conditions revealed a tetragonal structure that has not been previously observed in bulk MoO3 samples. Decreasing the growth temperature to 535 K led to polycrystalline alpha-MoO3 preferentially aligned with the [0 1 0] and [1 0 0] directions of the grains oriented normal to the substrate. By manipulating the initial growth conditions, relatively flat, epitaxial MoO3, films could be grown on SrLaAlO4,(0 0 1). In this case, the equivalent of one layer of beta-MoO3 (0 0 1) was deposited at 675 K before adding several layers of MoO3 at 550 K. Unlike growth solely at 675 K, the MoO3 formed in this manner did not dewet the surface when reheated to 675 K and RHEED indicated that continued MoO3 growth at this temperature proceeded epitaxially. XRD and AFM indicated that films grown in this manner contained alpha-MoO3, crystallites in addition to an epitaxial phase that accounted for most of the surface area of the film. STM images of the film that revealed step heights expected for P-MoO3 (0 0 1), along with RHEED results that revealed in-plane lattice constants consistent with beta-MoO3, indicated that epitaxial beta-MoO3. films can be grown and that geometric matching to the substrate can stabilize the beta phase at temperatures where it is not stable in the bulk.