Measurements of the reaction probabilities (gamma) for ClONO2 onto sulfuric acid solutions at 200-270 K are described. ClONO2 uptake due to reaction with H2O and with HCl in H2SO4 solutions (45-55 wt %) was investigated at 203-205 K. ClONO2 hydrolysis was also investigated on 36.5, 40, and 75 wt % sulfuric acid solutions over a range of temperatures. The measured gamma are generally in good agreement with previously reported values. In addition, the solubility of HCl was determined for 45 and 50 wt % sulfuric acid from 200 to 225 K. The uptake of ClONO2 onto small sulfuric acid particles was studied resulting in a lower limit to the sticking coefficient of 0.5. ClONO2 reaction probabilities were also measured on H2SO4 solutions containing significant amounts of HNO3. In opposition to previous reports, HNO3 was found to have a significant effect on gamma for ClONO2 : it is as low as one-half of that expected for the comparable HNO3-free solutions. The measured gamma on sulfuric acid solutions at 203 +/- 2 K are discussed in terms of solubility, diffusivity, and bulk and surface reactions. Within this framework, the measured gamma were fit as a function of H2SO4 and HCl content, thus allowing for the measurements to be extrapolated to atmospheric conditions.