The reaction of HNO3 with decane (C10H22) soot generated in the laboratory has been studied in a Knudsen flow reactor. Two different types of soot were produced: Soot originating from a rich flame of decane and air (“grey” soot) and soot generated from a leaner flame of decane and air (“black” soot). Both HNO3 uptake and product release have been observed. Whereas soot from a rich decane flame leads to HONO as the main product, NO and small amounts of NO2 are observed from soot generated in a lean flame. A reaction mechanism is proposed in which HNO3 is reduced to HONO, which is released into the gas phase on grey soot, but undergoes decomposition reactions on black soot to NO and NO2, the latter of which also reacts on black soot to produce additional NO. Experiments on black soot show that at [HNO3]>1011 molecule cm−3 a higher order process is induced which is associated with an additional production of NO and NO2. Further studies of the reaction of gaseous NO with adsorbed HNO3 reveal a slow reaction leading to HONO and NO2. Both initial uptake (γ0) of HNO3 as well as steady state uptake probabilities (γss) have been obtained: γ0=(4.6±2.3)×10−3 and γss=(5.2±1.3)×10−4 for reaction on grey soot and γ0=(2.0±0.1)×10−2 and γss=(4.6±1.6)×10−3 for black soot, using the geometric surface area over which the samples are spread.