a heteropolyacid catalyst for oxidation of isobutyraldehyde , methacrolein or mixtures or combinations thereof to methacrylic acid is disclosed where the heteropolyacid catalyst includes at least molybdenum , phosphorus , vanadium , and a first component including bismuth and / or boron . the heteropolyacid catalyst can also optionally include a second component including potassium , rubidium , cesium , and / or thallium and optionally a third component including antimony , cerium , niobium , indium , iron , chromium , cobalt , nickel , manganese , arsenic , silver , zinc , germanium , gallium , zirconium , magnesium , barium , lead , tin , titanium , aluminum , silicon , tantalum , tungsten , and / or lanthanum . the heteropolyacid catalyst can also include an ammonium - containing compound designed to increase a value of medium pores in the final heteropolyacid catalyst . a method for oxidizing isobutanal to methacrylic acid using the heteropolyacid catalyst is also disclosed .