The reactivity of Keggin-type P/Mo polyoxometalates (POMs) as heterogeneous catalysts for the selective oxidation of isobutane to methacrylic acid is reviewed, with reference to both industrial patents and scientific papers published in recent years. The main aspects of the reaction are discussed, and in particular the POM requirements which are necessary to develop an active and selective catalyst are examined for the compound (NH4)(3)PMo12O40. Particular attention is given to the importance of the reduction level of the POM in affecting the catalytic performance, and to the generation of the active sites in the reaction environment, in reference to the reaction conditions (either isobutane-rich or isobutane-poor). The discussion about the role of V in P/Mo/V POMs is also included.