This paper presents a thorough investigation of an optimised VMgO catalyst (14 wt% V) for the oxidative dehydrogenation of propane, carried out in order to elucidate the nature and behaviour of the active surface. The catalyst morphology and the surface composition are studied by means of HREM, XPS, UV-vis, XRD, and in-situ electrical conductivity techniques, as a function of the gaseous environments of the catalyst. The active surface is shown to be essentially a monolayer of amorphous VO43- units scattered over the magnesia as isolated and polymeric species. These surface vanadia units are found to stabilise an unusual polar (111) orientation of MgO up to temperatures of 800 degrees C. A direct and outstanding evidence of a totally reversible phenomenon of order/disorder restructuration of this V overlayer is provided in conjunction with the redox state of the surface depending on the properties of the surrounding atmosphere (reductive or oxidative). These fast surface phenomena are assumed to determine the elementary steps of propane activation within the overall ODHP process.