The speed of sound in gaseous 1,1,1,2-tetrafluoroethane (R-134a, CF3CH2F) and difluoromethane (R-32, CH2F2) has been measured by using a spherical resonator. The measurements for R-134a have been carried out along two isotherms at 323 K and 343 K and at pressures up to 400 kPa for a total of 26 values. For R-32 the measurements were made at 308 K, 323 K, 333 K, and 343 K and at pressures up to 500 kPa for a total of 44 measurements. The experimental uncertainties for R-134a in temperature, pressure, and speed of sound are estimated to be not greater than +/-6 mK, +/-0.2 kPa, and +/-0.0061%, respectively. The experimental uncertainties for R-32 in temperature, pressure, and speed of sound are estimated to be not greater than +/-8 mK, +/-0.2 kPa, and +/-0.0061%, respectively. The purities of the R-134a and R-32 : samples were better than 99.95% and 99.99% of area percent of the gas chromatography, respectively. We have determined the ideal-gas heat capacities and the second acoustic virial coefficients from the speed-of-sound measurements.