We report new results of Differential Thermal Analysis (DTA) of crystallization of impure amorphous H2O ice (a-H2O), containing CO, CO2 and CH4 molecules. It is revealed that the heat of crystallization of impure a-H2O is reduced by the impurities. The crystallization is not exothermic like pure a-H2O, but endothermic when the concentration of impurities is higher than 2% regardless of molecular species. An out-gassing rate from a model cometary nucleus is computed numerically using the experimental values. The result shows that the runaway crystallization of cometary ice never occurs and that the mechanism of outbursts should be reexamined. ¿ 2001 American Geophysical Union