The temperature (T) dependence of the Cl- yield in the electron-stimulated desorption (ESD) of a submonolayer of CF2Cl2 adsorbed on amorphous H2O ice grown at 25 K exhibits a peak between 50 and 70 K. This peak and a sharp drop in electron-trapping coefficient of ice near 50 K show strong evidence for a phase transition. The peak disappears for CF2Cl2 on crystalline ice; instead, a non-Arrhenius Cl-dependence on Tis observed with a critical temperature of similar to 224 K. The peak is also not pronounced in the T dependence of the ESD H- yield of amorphous ice, which involves no transfer of trapped electrons.