The performance of a vapor compression heat pump system is tested using R22/R114 mixtures as the working fluid. Double pipe heat exchangers, where the refrigerant flows inside the inner tube with grooved internal surface and water flows countercurrently in the outer annulus, are used for the condenser and evaporator. Experimental results are compared with those for the same system with a smooth tube condenser and the evaporator presented before. The effect of the superheating of the refrigerant at the exit of the evaporator on COP is also discussed. A trial calculation based on the experimental results shows that the COP of the mixture becomes larger than that of the pure refrigerant in the case of a large change in the water temperature only if the heat transfer surface area is large enough and the superheating of the refrigerant is quite small at the exit of the evaporator.