By measuring laminar recovery factors in a high velocity gas stream, experimental determinations were made of the Prandtl number of carbon dioxide over a temperature range from 285 to 450 K and of carbon-dioxide air mixtures at an average temperature of 285 K with a predicted maximum error of 1.5 per cent. Thermal conductivity values were deduced from these Prandtl numbers and compared with literature values measured by other methods. Using intermolecular force constants determined from literature experimental data, viscosities, thermal conductivities, and Prandtl numbers were calculated for carbon-dioxide air mixtures over the temperature range 200 to 1500 deg for mixture ratios from pure air to pure carbon dioxide.