Methyl bromide was synthesized by reacting methane with oxygen and hydrogen bromide over Rh/SiO2 catalyst. The reaction started from the oxidation of HBr to form active bromine species (Br-center dot radicals and Br* surface species), which in turn reacted with CH4 to form CH3 center dot radicals and CH*(3) surface species. These CH3 center dot and CH*(3) species reacted with the active bromine species to form CH3Br and CH2Br2. The presence of HBr inhibited the deep oxidation and the steam reformation of CH4 and therefore, guaranteed the high selectivity of CH3Br. In the presence of HBr, CO was formed from the oxidation and steam reformation of CH3Br, while CO2 was formed from the oxidation and steam reformation of CO over Rh/SiO2 at reaction temperature higher than 560 degrees C.