The speed of sound in gaseous propane has been measured along seven isotherms in the temperature range 225 K to 375 K at pressures up to the lower of 0.85 MPa and 80 per cent of the vapour pressure. The results have a precision of 0.001 per cent or better and an overall uncertainty, including an allowance for possible undetected impurities, of 0.01 per cent. Isobaric perfect-gas heat capacities and second and third acoustic virial coefficients have been determined from the results.