The reaction between molecular HOCl and a chlorinated surface of ice to produce molecular chlorine has been theoretically investigated. Ab initio quantum mechanical tools (GAUSSIAN 98 ) have been used for this purpose, based on a molecular cluster and on an embedded cluster model, respectively. Different reaction paths have been explored, requiring a pre-activation of the substrate. In all cases, important activation barriers have been found. Only if certain types of structural defects are present at the surface, the reaction has been found to take place almost spontaneously.