An experimental study of the pressure-volume-temperature-composition (PVTx) properties in the gas phase for binary mixtures of 2,3,3,3-tetrafluoropropene (HFO-1234yf)+ 1,1,1,2-tetrafluoroethane (HFC-134a) was conducted in the range of temperatures from 298.58 to 403.24 K, pressures from 567.5 to 3171.2 kPa, densities from 0.258 to 1.258 mol dm(-3), and compositions from 0.0380 to 0.8641 mole fractions of HFC-134a. The measurements were performed with an isochoric cell apparatus. The uncertainties in the present work were estimated to be +/- 1.5 kPa for pressure, +/- 6 mK for temperature, and +/- 0.15% for composition. On the basis of the experimental PVTx property data, a truncated virial equation of state was developed for the binary HFO-1234yf/HFC-134a system. This equation reproduced the experimental data in the gas phase within +/- 0.20% in pressure and within +/- 0.23% in density. (C) 2014 Elsevier B.V. All rights reserved.