The uptake of HI and co-uptake of HI + HCl on water-ice have been studied in a flow reactor at polar stratospheric temperature. The uptake of HI on ice is determined to be in the range of 1.1 x 10(14)-1.8 x 10(16) molecules/cm(2). The HI uptake thermodynamics is also investigated. The nature of higher uptake and the effect of acidity on the HX uptake are studied and discussed. The HI + HCl co-uptake experiment reveals that the uptake of HI on ice is about a factor of 10 more efficient than that of HCl at 188 K.