Soot samples from a spark generator, a flame, and a diesel passenger car were either collected on a Teflon filter and transferred to an IR-transparent window or deposited directly from a flame onto the window and investigated by Fourier transform infrared (FTIR) spectroscopy. The soot-covered windows were mounted in a 10 cm vacuum cell connected to a standard flow system with He as carrier gas. Reactive gases, such as NO2 and HNO3, were added to the carrier gas flow at a concentration of (0.016 to 2.5) × 1014 molecule cm-3. FTIR spectra of soot samples before and after exposure to HNO3, NO2, and O3 are presented. Formation of IR absorption bands was analyzed as a function of exposure time. IR bands attributable to soot surface oxidation products and nitrogen containing species, e.g. −CO, R−NO2, R−ONO2, and R−ONO were observed. The observed time dependence of the absorption bands of the spark generator soot can be fitted by two parallel reactions, a slow and a fast process. Both processes have a reaction order of n ≈ 0.2 (±0.3) for the NO2 + soot reaction and n ≈ 0.5 (±0.6) for the HNO3 + soot reaction. The number of active sites, N max = 2.2 × 1014 molecules cm-2 soot surface, has been estimated from saturation experiments. Surface reaction probabilities depend on reactant concentration and reaction time and were in the range of γ ≈ 10-6 to 10-8 for the slow, and γ ≈ 10-3 to 10-6 for the fast processes. The reaction probability on diesel engine soot was nearly 1 order of magnitude slower. It is concluded that the reaction of NO2 with soot cannot account for the HONO levels observed in urban air.