The reactive and physical uptakes of HNO3, O3, and N2O5 were examined on hydrocarbon soot by using a flow tube equipped with a chemical ionization mass spectrometer. The uptake of HNO3 at 298 K was found to be reversible, and it did not produce NO2 or NO; i.e., HNO3 was not reduced. The uptake of O3 on to soot was measured at 298 and 253 K and found to be small when the surface area of the soot was taken into account. We conclude that neither the HNO3 nor the O3 reaction on soot has much impact on the atmosphere. We observed the production of gas phase NO2 from the uptake of N2O5 on the soot surface with an efficiency of close to unity. It was concluded that the uptake of N2O5 on soot is reactive. The implications of the N2O5 reaction on soot to the atmosphere are not clear, because not all the products of this reactive uptake have been identified ¿ 2000 American Geophysical Union