Keggin-type molybdophosphoric heteropolyacid. catalysts with iron cation partially substituted for proton and arsenic for phosphorus in a bulk form have been synthesized for the selective oxidation of isobutane into methacrylic acid (MAA). It was found that the addition of iron can obviously improve both the conversion of isobutane and selectivity to MAA, while arsenic substitution was mainly in favor of the selectivity to MAA. The best conversion and yield of MAA were 23.9% and 16.7%, respectively, which was obtained at 370degreesC with a space velocity of 420 h(-1) over HxFe0.12PVAs0.3Mo11 O-y heteropoly compound catalyst. The catalysts were characterized by XRD, SEM, FIF-IR, and TPR. (C) 2004 Elsevier B.V. All rights reserved.