Uptake experiments at ambient temperature of NO2 on ethylene, acetylene and toluene soot resulted in significant amounts of HONO. The initial uptake coefficient &ggr;0 of NO2 on ethylene soot decreased from 0.12 to 0.03 with increasing NO2 concentration in the range 8¿1010 to 1.4¿1013 molecule s-1. The HONO yields relative to NO2 taken up were in the range 50 to 93% and were consistent with a mechanism in which either soot or hydrogen contained in it was the reducing agent. The branching ratio of the yield of HONO and NO was a function of the sampling spot at which the soot was collected within a diffusion flame and ranged from 23 to 2.2 with increasing distance from the flame base and thus age. The total amount of HONO generated in one uptake experiment was 1.2¿1016 molecules per mg of soot. Heating cycles bring back the ability of soot to generate HONO, but at diminishing yields. ¿ 1998 American Geophysical Union