The speed of sound u has been measured in {(1 - x(2))C2H6 +x(2)CO(2)} with x(2) approximate to 0.6, along seven isotherms at temperatures between T = 220 K and T = 450 K by means of a spherical resonator. The greatest pressure on each isotherm was approximately p = 1.2 MPa except at T = 220 K where, to avoid condensation, it was restricted to p = 0.6 MPa. Corrections to the speed of sound for the effects of vibrational relaxation have been calculated from a two-mode model containing relaxation times that were fitted to the excess sound absorption in the mixture and in both pure components. The corrected speeds of sound have a precision better than fl 10(-5). u and an estimated overall uncertainty of +/-1.10(-4) u. The precise composition of the sample used on each isotherm was determined from the experimental speeds of sound extrapolated to the limit of zero pressure. Second and third acoustic virial coefficients of the mixture and ordinary interaction second virial coefficients are reported.