The heterogeneous reactivity of NO2 with soot, two combustion products typically produced in close proximity, may have significant implications for the chemistry occurring in the atmosphere, especially the urban troposphere. As the soot produced in the environment is not well characterized, the interaction of NO2 with a number of different types of laboratory-generated soot was studied. The production of HONO was observed in the interaction of NO2 with methane, propane, hexane, and kerosene soot. It was found that soot produced from hydrocarbons catalyzed the reaction of NO2 with H2O to give HONO. Reactive uptake coefficients for this process that are measured in the laboratory may not be easily applied to atmospheric calculations without more information. ¿ 1999 American Geophysical Union