The oxidative conversion of propane to acrylic acid and acetic acid over Mo-V-Sb-R-O (R=La, Cc, Nd and Sm) catalysts at different reaction conditions (viz. temperature, C3H8/O-2 ratio, H2O/C3H8 ratio, space velocity, etc.) was investigated. The catalytic activity and selectivity of the Mo-V-Sb catalyst are strongly influenced by the addition of rare earth metals to the catalyst. The addition of water vapour to the feed of propane and oxygen enhances greatly the formation of oxygenated products, particularly acrylic acid and acetic acid. The ratio of selectivities of acrylic acid to acetic acid was found to depend on the rare earth metal used in the catalyst's preparation and the reaction conditions. High contact times, i.e. high degrees of propane conversion, are detrimental to the formation of acrylic acid but beneficial for acetic acid formation. (c) 2005 Society of Chemical Industry