The speed of sound u in gaseous ethane of mole-fraction purity 0.9999 has been measured along 17 isotherms al temperatures between 220 K and 450 K. At temperatures of 300 K and above, the greatest pressure on each isotherm was chosen to correspond to that at approximately one half of the critical density. Below T = 300 K, the greatest pressure on each isotherm was limited to that al which the estimated density was about 0.8 of the saturated vapour density. The measurements were made using a spherical acoustic resonator and have an estimated total uncertainty not greater than about 1 . 10(-4). u; this uncertainty is dominated by the possibility that the sample contained undetected impurities. Second and third acoustic virial coefficients have been obtained from the results and used to determine parameters in model two-and three-body intermolecular potential-energy functions for ethane. Ordinary second and third virial coefficients calculated from these models are reported. Moreover, a numerical integration of the equations that link u with the other thermodynamic properties of the fluid has been performed inside the region where the speed of sound was measured. The maximum uncertainty of the thermodynamic properties derived in this way is estimated to be 0.04 per cent for the compression factor and 0.4 per cent for the isobaric and isochoric heat capacities.