The kinetics of reaction between silicon nanoparticles and molecular oxygen were studied by tandem differential mobility analysis. Aerosolized silicon nanoparticles were extracted from a low-pressure silane plasma into an atmospheric pressure aerosol flow tube reactor. Particles were initially passed through a differential mobility analyzer that was set to transmit only those particles having mobility diameters of approximately 10 nm. The monodisperse particle streams were mixed with oxygen/nitrogen mixtures of different oxygen volume fractions and allowed to react over a broad temperature range (600-1100 degrees C) for approximately one second. Particles were size-classified after reaction with a second differential mobility analyzer. The particle mobility diameters increased upon oxidation by up to 1.3 nm, depending on the oxygen volume fraction and the reaction temperature. Oxidation is described by a kinetic model that considers both oxygen diffusion and surface reaction, with diffusion becoming important after formation of a 0.5 nm thick oxide monolayer.