Carbon dioxide (R744) and propene (R1270) are expected to be long-term working fluids for air conditioning systems, heat pumps, and organic Rankine cycles (ORCs) because of their low global warming potentials (GWP). The speed of sound was measured in gaseous carbon dioxide from (260 to 333) K and in gaseous propene from (260 to 330) K using a cylindrical resonator at pressures up to 1 MPa. The perturbations from the thermal and viscous boundary layers, the gas fill duct, the shell motion, and the vibrational relaxation were corrected in the frequency measurements. The uncertainties in the temperature, pressure, and speed of sound measurements were estimated to be less than 5 mK, 200 Pa, and 0.02 %. The ideal-gas heat capacities at constant pressure and the acoustic virial coefficients of carbon dioxide and propene were deduced from the measured speed-of-sound data. The second virial coefficients for carbon dioxide and propene were obtained from the acoustic data and the square-well intermolecular model.