Heat transfer characteristics of a potential refrigerant R 32/R 1234ze(E) in horizontal microfin tubes are experimentally investigated in this study. The heat transfer coefficient (HTC) and pressure drop are measured at saturation temperatures 40 and 10 °C for condensation and evaporation, respectively. The HTC of R 1234ze(E) is lower than that of R 32, as predicted by several correlations. Degradation in the HTC of the R 32/R 1234ze(E) mixture is significant; the HTC is even lower than that of R 1234ze(E). During condensation and evaporation process, the HTC is minimized at compositions 0.5/0.5 and 0.2/0.8 by mass fraction, respectively. The magnitude of heat transfer degradation by the volatility differences is much severer for evaporation than condensation.