The sizing of evaporators for refrigeration and air conditioning applications is essential for good cycle performance. For this reason, accurate knowledge of the heat transfer coefficients and pressures drops of refrigerants is mandatory. In order to identify the best substitute for R22, in this paper, the experimental local heat transfer coefficients and pressure drops during flow boiling of R407C and R417A were compared.The test section was a smooth, horizontal, stainless steel tube (6.0mm I.D., 8.0mm O.D., 6.0m length) uniformly heated by the Joule effect. The experimental tests were conducted by varying the evaporating pressures within the range from 3.6 to 10.3bar, heat fluxes within the range from 7.6 to 28.5kW/m2 and refrigerant mass fluxes within the range from 195 to 706kg/m2s.