The catalytic performance of Cs-substituted phosphomolybdic salts was studied for selective oxidation of isobutane.The results of activity tests revealed that 360 ℃ was the optimal reaction temperature.It was demonstrated that oxidizing sites not only took dominating part in the activation of isobutane,but also influenced the product distribution.Besides,appropriate Cs addition led to moderate acidity of catalysts,favoring the selectivity to desired products.Furthermore,to obtain partially reduced catalysts,different calcination atmospheres were investigated and certain proportion of Mo5+ produced during calcination was crucial for the redox reaction.The catalyst calcined in N2 showed the highest yield of MAA (7.0％).Fe-substitution enhanced the activity of catalysts by rapid reoxidation of Mo5+.