The effect of water vapor on hydrocarbon combustion (CH4, C2H4, C3H8) was studied in the presence of an HFC agent (HFC-125). The effect depends on the F/H ratio of the initial mixture. A promotion effect was observed in mixtures with the F/H ratios ranging approximately from 0.9 to 2. The calculated maximum increase in peak flame temperature was in the range of 100-150 K. and in burning velocity, in the range of 1-2 cm/s. The change of the ratio from F/H ratio <1 to the conditions with the F/H ratio >1 corresponds to the disappearance of H2O and a substantial increase of CF2O in the combustion products. Thermodynamic and laminar premix flame calculations demonstrate that "extra" fluorine, which is in excess of hydrogen (F/H > 1), reacts with added H2O forming HF molecules. Calculations demonstrate that the equilibrium volume fractions of the fluorine atom can be as large as 0.5-3% for mixtures with an F/H > 1. The main reaction of H2O conversion to HF is the F + H2O = HF + OH reaction. Dependencies of the F/H ratio as a function of HFC-125 (C2F5H) concentration and showing the possible range of mixture compositions for a promotion effect, were generated for methane, ethylene and heptane at different equivalence ratios. Published by Elsevier Inc. on behalf of The Combustion Institute.