One hundred ninety-three speed-of-sound values in gaseous difluoromethane (R-32, CH2F2) + 1,1,1,2-tetrafluoroethane (R-134a, CF3CH2F) have been measured using a spherical resonator. The measurements have been carried out at temperatures from 303 K to 343 K, pressures up to 240 kPa, and mole fractions of R-32 from 0.16 to 0.90. The experimental uncertainties in the temperature, pressure, and speed of sound for the binary mixture are estimated to be not greater than +/-8 mK, +/-0.1 kPa, and +/-0.0072%, respectively. The samples purified and analyzed by the manufacturers were used and were better than 99.99 mass % for R-32 and 99.98 and 99.99 mass % for two different R-134a samples. We have accurately determined the compositions of the binary refrigerant mixture, R-32 + R-134a, and the second acoustic virial coefficients from the speed-of-sound measurements.