Iron oxide thin films were deposited onto various substrates under various O-2 how rates by metal-organic chemical vapor deposition. Spinel Fe3O4+x was formed at lower O-2 flow rates on all substrates. However, at higher O-2 how rates, a mixture of beta-Fe2O3 and spinel Fe3O2+x was formed on a non-alkaline glass and the (100) plane of SiTiO3 single-crystal substrates, and highly (100) oriented spinel Fe3O4+x was formed on (100) MgO single crystals and on (100) oriented MgO buffer layers. Mainly (311) oriented spinel Fe3O4+x was formed on (111) oriented MgO buffer layers. The experimental results are discussed with respect to the lattice mismatch between spinel Fe3O4+x and the substrates.