A measuring device for measuring the mass flow of nonsteady-state two-phase flows has been developed based on the principle of rotating flow machines. Two versions of the device [true mass flowmeter (TMFM) 2.5 with a measuring range of 2.5 kg/s, and TMFM 50 with a measuring range of 50 kg/s] were used to study the measuring accuracy and the field use of the system. While the measurement errors of TMFM 2.5 are within ±2.2% of the maximum flow, it is possible with the TMFM 50 to reduce the measuring error to ±1.5% (quality x<l%) and ±0.5% (quality ≥ 1%). This implies that the accuracy in measuring two-phase mass flow is practically identical with that obtained in single-phase flow by familiar standard measuring techniques.