The uptake of ClONO2 onto sulfuric acid solutions and HCl-doped sulfuric acid solutions has been measured over temperature and composition ranges relevant to the high-latitude lower stratosphere. The results reveal that the reaction of ClONO2 with HCl in/on sulfuric acid is an important source of active chlorine for the stratosphere. It is suggested that ClONO2 uptake due to reaction with HCl is dependent on both bulk and surface concentrations of HCl and that, for present levels of HCl in the stratosphere, the surface-dependent uptake could be a significant contributor to the conversion of ClONO2 and HCl to Cl-2 at temperatures less than 195 K, if type I PSCs are not formed at their threshold temperatures. The hydrolysis of ClONO2 and the gamma for uptake in the absence of HCl have been shown to be dependent on the activity of water. The results suggest a larger reactive uptake at temperatures near 200 K than has been used previously. The parameters obtained from these uptake measurements will lead to better representations of the ClONO2 + H2O and ClONO2 + HCl reactions in atmospheric models.