Cesium and copper- or iron-substituted Keggin-type phosphomolybdic heteropolycompounds with the general formula Cs2Mxy+H1-xyPMo12O40 (M = Fe3+ and Cu2+ and 0 less than or equal to x less than or equal to 0.43) have been tested as catalysts in the oxidation of isobutane to methacrylic acid in order to study the effect of the transition metal ion on the catalytic properties. Copper appears to have a positive effect on the activity and a negative effect on the selectivity in methacrylic acid. Iron has almost no effect on the activity but a positive one on the selectivity, although this trend is reversed at higher iron content levels. These results have been correlated to the reducibility and the acidity of the catalysts.