A chemical potential diagram of the V-Mg-O system was constructed for comparison in an in-situ experiment. A V-Mg-O catalyst used in the oxidative dehydrogenation of n-butane was prepared by the impregnation method and was characterized by in-situ X-ray diffraction (XRD). Mg3V2O8 and MgO were detected on the in-situ XRD pattern under the oxygen pretreatment at 600 A degrees C, and the in-situ XRD data under working conditions showed that Mg3V2O8 is reduced to MgV2O4, having a cubic spinel structure with a lattice constant of a = 8.427 A.... The observed reduction of V5+ in Mg3V2O8 to V3+ in MgV2O4 under the working conditions could be well understood through a chemical potential diagram.