A new method of measuring the steam quality based on ultrasound is presented in this paper. This method is based on rigorous consideration of the steam＇s thermodynamic properties. It is shown in the paper that the steam＇s speed of sound depends on its quality at a given saturation temperature. Thus, the quality of the wet steam can be determined if ifs speed of sound is known. The speed of sound of a flowing fluid can be measured using transit-time ultrasonic meters. Field testing of a pair of custom-designed ultrasonic transducers has shown that it is feasible to measure the steam＇s speed of sound. An uncertainty analysis of this new method is also presented in the paper The analysis suggests that this method is capable of achieving a measurement accuracy of better than +/-3 percentage points of the steam quality under typical oil field steam injection conditions.