Supporting extremely small concentrations of noble metals on SiO2 and Bi2Mo3O12 affected the catalytic activity and product distribution in iso-butane partial oxidation. Supporting Pt, Pd, and Rh onto SiO2 by 1 atom/mum(2) enhanced the catalytic activity. Selectivity values of partial oxidation products, iso-butene, CH3C(CH3)CHO, CH3CHO and CH3COCH3, were increased and COx selectivity was decreased. Rh/SiO2 gave an EPR signal due to isolated Rh2+ ions with g(perpendicular to) = 2.07 and g(parallel to) = 1.95, suggesting that two valent Rh ions are well dispersed on the surface and play a role in the oxidation of iso-butane. Supporting Pd, Ir, Rh, and Au onto Bi2Mo3O12 at a concentration of (1/9) x 10(-4) atom/nm(2) increased the conversion, but supporting Pt and Ru decreased it. Selectivity values of CH2=C(CH3)CHO and COx were the same for every catalyst. Catalytic activity was increased by supporting them by (1/49) x 10(-4) atom/nm(2); especially, supporting Rh increased the activity without changing the selectivity of CH2=C(CH3)CHO. Supported noble metal will interact with active oxygen ions bridging Mo and Bi on the Bi2Mo3O12 surface. Since a certain concentration of supported noble metals, (1/49) x 10(-4) nm(2), begins to affect the catalytic activity and selectivity, this concentration may reflect the concentration of active sites in iso-butane oxidation at 350-400degreesC over Bi2Mo3O12.