Thin-film solid-state reactions in the system MgO-Nb2O5 are experimentally investigated. MgO (001) substrates are subjected to Nb-O vapor at different temperatures in high vacuum. Thin films containing the phases Mg4Nb2O9 and MgNb2O6 are formed by a vapor-solid reaction between the Nb-O vapor and the substrate. The crystallographic orientations of these phases are studied by X-ray diffractometry including pole figure analysis. Mg4Nb2O9 grows (11.4)-, (11.6)-, and (11.9)-oriented, whereas MgNb2O6 grows with a preferential (241) orientation. The crystallographic relationships and their origins are discussed.