The measured reaction probability, gamma, for the reaction of chlorine nitrate (CIONO2) with 60 percent (by weight) sulfuric acid aerosol increases monotonically with particle size at 250 kelvin. The reacto-diffusive length (l, the effective liquid depth over which reaction occurs) derived from these experiments is 0.037 +/- 0.007 micrometer (95 percent confidence level for precision). The reaction probability for the reaction of CIONO2 with 60 percent sulfuric acid aerosol doped with similar to 7 x 10(-4) M hydrochloric acid at 250 kelvin is larger by about a factor of 4 than in the absence of hydrochloric acid and varies less with particle size (l = 0.009 +/- 0.005 micrometer), These results provide a test of the theory for gas-particle reactions and further insight into the reactivity of atmospheric aerosol.