The vapour phase partial oxidation of p-methoxytoluene to p-methoxybenzaldehyde was investigated over various V2O5/CaO-MgO catalysts at 673 K and under normal atmospheric pressure. Among various catalysts the 14% V2O5/CaO-MgO sample exhibited good conversion and product selectivity. The CaO-MgO mixed oxide support was prepared by a homogeneous co-precipitation method using urea as hydrolysing agent. Ammonium metavanadate was used as source of V2O5 and the vanadia contents were varied from 2 to 14 weight per cent. The CaO-MgO support and various V2O5/CaO-MgO catalysts were characterized by means of X-ray diffraction, FT-infrared, electron spin resonance, scanning electron microscopy, NH3 and CO2 chemisorption, and BET surface area methods. Characterization results suggest that the vanadia does not form layer structures on the support surface, instead but interacts very strongly with the support oxide, in particular with MgO, and forms amorphous compounds. The NH3 and CO2 uptakes clearly distinguish the acid-base characteristics of the catalysts and correlate with catalytic properties.