Phosphomolybdate compounds have been investigated for their structural characteristics and oxidation activity toward isobutane. The phosphomolybdates were synthesized from phosphomolybdic acid and the fourth period transition metal cations Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+ Cu2+, Cu+ and Zn2+. Two compounds were synthesized per transition metal: where (i) all the protons had been replaced by the particular transition metal, and (ii) only partial proton replacement leaving a single proton per phosphomolybdate. X-ray diffraction analysis has shown that a primitive cubic phase is apparent with some of the transition metal phosphomolybdates. Each solid was exposed to isobutane using the anaerobic low-pressure steady-state technique. Category 1 exponential-like distributions of methacrolein were observed with all the transition metal phosphomolybdates, except the lower oxidation state iron and copper salts, Fe-1.5[PMo12O40] and Cu-3[PMo12O40] respectively. Activation energies ranged from 51.31 +/- A 0.27 kJ mol(-1) (Cr[PMo12O40]) to over 200 kJ mol(-1) (Zn-1.5[PMo12O40]). Phosphomolybdates with counter cations which are one or two electrons deficient from either a 3d (5) or 3d (10) configuration (in this case 3d (3), 3d (8) or 3d (9)) had the lowest activation barriers for methacrolein formation. A computational investigation into HOMO-LUMO band gap energies agrees with the association. The presence of protons also enhances Category 1 product formation and may be attributed to migration of H+ through the bulk of the solid.