The catalytic dehydrogenation of ethyl alcohol on vanadium pentoxide supported on magnesia was studied in a flow system. The catalysts were characterised by DTA, X-ray, surface area and electrical conductivity measurements. The results revealed that the yield of acetaldehyde is strongly affected by the nature of compounds formed as a result of the interaction between MgO and V2O5. The Mg3V2O8 formed at a calcination temperature of 550 degrees C is the most active and selective catalyst. The activity of this spinel was correlated with the creation holes and the position of the Fermi potential. On the other hand, the catalyst calcined at 650 degrees C was accompanied by the formation of an MgV2O6 spinel, which exhibited low activity and low selectivity.