Eight lanthanum- (La0.25H2.25[PMo12O40], La0.5H1.5[PMo12O40], La0.75H0.75[PMo12O40], La[PMo12O40]) and cerium- (Ce0.25H2.25[PMo12O40], Ce0.5H1.5[PMo12O40], Ce0.75H0.75[PMo 12O40], Ce[PMo12O40]) containing phosphomolybdate catalysts have been synthesized and analysed using a low pressure steady state technique. The products from isobutane oxidation using the catalysts were water, methacrolein, carbon dioxide and lactone. Methacrylic acid is not detected under the low-pressure molecular-flow conditions; lactone is postulated to be intermediary in methacrylic acid production. The methacrolein and lactone data were simulated using two different theoretical models in order to determine kinetic parameters. The activation barriers for methacrolein formation vary substantially throughout each lanthanum and cerium series, however the most active catalyst is determined to be Ce[PMo12O40]. Only three of the eight catalysts produced significant quantities of lactone (La0.75H0.75 [PMo12O40], La[PMo12O40] and Ce[PMo12O40]) and this activity is correlated with the most active catalysts for methacrolein formation. The trends in selective oxidation activity must be due to the acidity and redox properties, as well as the structural phases, but it is difficult to quantify in all cases.