The reactive uptake of ClONO2 on liquid sulfuric acid has been measured for size-selected, sub-micron aerosol in the composition range 36--54% H2SO4 by weight at ≈250 K. A comparison with results from studies employing bulk surfaces at similar temperatures and compositions shows no evidence for a dependence of the reaction probability, &ggr;, on size. We use this observation to estimate an upper limit to the diffuso-reactive length of 26 nm for the ClONO2+H2O reaction on 43 weight % aerosol at 248 K. But, the present values of &ggr; are consistently lower than bulk values observed at lower temperatures and similar composition, suggesting that &ggr; is temperature dependent. ¿ 1998 American Geophysical Union