The economized vapor injection (EVI) method is used to decrease the discharge temperature of scroll compressors and to improve the cycle performance of vapor compression refrigeration systems by directly injecting refrigerant vapor into the compression chamber. In previous studies, the optimal cycle performance for an air conditioning system using EVI was detected during two-phase injection. However, the exact vapor mass fraction could not be measured directly. For further investigations on two-phase injection systems, accurate vapor mass fraction (VMF) measurements need to be conducted. This paper compares different options for the VMF measurement using pure refrigerants or refrigerant mixtures. It focuses on the achievable accuracy of the VMF measurement, based on the propagation of uncertainty.