The ER‐2 data from ascents and descents through layers of cirrus clouds are utilized to study the heterogeneous reactions of ClONO₂ with H₂O, of HOCl and ClONO₂ with HCl, and their potential role for the activation of chlorine in the tropopause region, which could affect ozone there. Lacking measured data for the three chlorine containing molecules their abundances as a function of altitude have been calculated from a 2D model. The aerosol surface data measured by a Forward Scattering Spectrometer Probe (FSSP‐300) on the ER‐2 were corrected for the expected asphericity of cirrus cloud particles by means of a T‐matrix method. The results indicate considerable potential of cirrus clouds for chlorine activation. If ClONO₂ and HCl are present in the tropopause region they are likely to be quickly converted to active chlorine by cirrus clouds.