A new experimental method has been developed to conduct surface chemistry and extract surface kinetic rates on size-selected nanoparticles. The method utilizes a tandem differential mobility analyzer (TDMA) technique in which monodisperse particles are selected from a polydisperse aerosol input stream and then subjected to chemical processing. The change in particle size is measured and used to determine kinetic information for the relevant surface reaction. The method has been applied to measure the oxidation rate of soot in air over the temperature range 800-1120°C. Soot was generated in situ using an ethylene diffusion flame and sent to a differential mobility analyzer (DMA) to extract monodisperse particles. Three initial particle sizes of 40, 93, and 130 nm mobility diameter were subjected to oxidation in a high-temperature flow reactor, and the resulting change in particle size was measured with a second DMA. The measured size decreases were fit to a model utilizing a modified Arrhenius expression for the rate of decrease: Ḋp = -AnmT1/2 exp(-Ea/(RT)). The fit yielded an activation energy of Ea = 164 kJ mol-1 with a different preexponential factor, Anm, for each initial particle size. The size-decrease rates, and therefore the preexponential factors, differed by a factor of 1.7 between the 40 and 130 nm particles, with the 130 nm particles decreasing faster than the 40 or 93 nm particles. This may be the result of several factors including different effective densities or different soot particle compositions. The current experiments are the first measurements of the soot oxidation rate to be performed on size-selected, freshly generated soot particles using online aerosol techniques. Our results agree well with previous work over the temperature range covered, which is somewhat surprising given the wide range of techniques and materials previously studied in the effort to understand soot oxidation.