The Cl- yield produced by dissociative electron attachment of a submonolayer of CF2Cl2 is enhanced by factors of 10(2) and 10(4) when CF2Cl2 is coadsorbed with water ice and ammonia ice, respectively, on a surface at similar to 25 K. Moreover, the magnitude of Cl- enhancement increases strongly with decreasing CF2Cl2 concentration. This enhancement is attributed to dissociation of CF2Cl2 by capture of electrons self-trapped in polar water or ammonia molecules. This process may be an unrecognized sink for chlorofluorocarbons in the atmosphere. Cl- ions produced may be directly or indirectly converted to Cl atoms, which then destroy ozone.