The kinetics of the partial oxidation of methane to formaldehyde (MPO) on a "precipitated" silica catalyst in the 500-800 degreesC range was investigated. The influence of temperature and reaction mixture composition on the density of reduced sites of the catalyst under steady state was evaluated by in situ reaction temperature oxygen chemisorption measurements. The kinetics of the MPO on silica is pseudo first and zero-order on P-CH4 and P-o2, respectively, while the density of reduced sites depends upon the square root of the P-CH4/P-O2 ratio. The rate-determining step is the activation of C-H bond of CH4 molecules (E-red = 142 kJ.mol(-1)), while oxygen replenishment is a nonactivated reaction step (E-ox approximate to 20kJ.mol(-1)), A Langmuir-Hinshelwood kinetic model accounting for a two-site dissociative activation mechanism of both CH4 and O-2, thoroughly describes the steady state of the silica surface and the reaction kinetics of the MPO in the 500-800 degreesC range.