A method based on an orifice flow equation is provided for determining both the quality and the mass flow rate of steam flowing in a line, such as to an injection well in steam flooding operations for recovery of petroleum. A sample of the steam is drawn off from the steam line through an orifice and is condensed and collected. The mass of sample collected and the time to collect it give the mass flow rate of the sample. Pressure and temperature measurements at the sample line orifice together with the measured mass flow rate of the sample and with known constants are used in the orifice flow equation to derive the sample steam quality. The line steam also flows through an orifice, with pressure and temperature measurements thereon. The desired quality of the line steam is the same as the derived quality of the sample steam, since care is taken to make them the same. The orifice flow equation is used again, substituting the derived sample steam quality for the line steam quality, together with the temperature and pressure measurements at the steam line orifice and with known constants, to derive the desired mass flow rate of the line steam.