The speed of sound u in carbon dioxide has been measured along seven isotherms at temperatures between (220 and 450) K by means of a spherical resonator. At supercritical temperatures, the greatest pressure on each isotherm was chosen to correspond to approximately one-half of the critical density while, at subcritical temperatures, the greatest pressure on each isotherm was limited to that at which the density was about 80 per cent of the saturated vapour density. Vibrational relaxation, which is severe in pure carbon dioxide at frequencies of the order 10 kHz when the pressure is below p = 1 MPa, necessitates significant corrections for dispersion and leads to greatly enhanced sound absorption, which limits the precision of the measurements somewhat. Nevertheless, the results have an overall estimated uncertainty which varies from 3.10(-4) u at the lowest temperature and pressure studied, to better than 5.10(-5) u at p greater than or equal to 1 MPa. The results have been analysed to obtain both second and third acoustic virial coefficients, and ordinary second virial coefficients.