This paper presents essentially measurements of ultrasonic speed u and phase transition in a ternary mixture made up of methane, propane, and octane, envisaged in the gaseous state. The ultrasonic-speed measurements were carried out in the temperature range from 293.15 K to 373.15 K and at pressures from the phase boundary up to 100 MPa with a total uncertainty of less than 0.1 per cent. Comparisons between the results and the predictions of several equations of stare, widely used in the petroleum industry, are reported and discussed.