An experimental study of the pressure-volume-temperature-composition (PVTx) properties in the gas phase for binary mixtures of ethyl fluoride (HFC-161) and 1,1,1,2-tetrafluoroethane (HFC-134a) was conducted in the range of temperatures from 318.22 to 403.20 K, pressures from 1120.6 to 5767.0 kPa, densities from 0.494 to 3.974 mol dm(-3), and compositions from 0.3203 to 0.9474 HFC-161 mole fractions. The measurements were performed with an isochoric cell apparatus. The uncertainties in the present work were estimated to be +/- 1.5 kPa for pressure and +/- 6 mK for temperature. On the basis of the experimental PVTx property data, a truncated virial equation of state was developed for the HFC-161/134a binary system. This equation reproduces the experimental data in the gas phase within +/- 0.18% in pressure and within +/- 0.32% in density. (C) 2011 Elsevier B.V. All rights reserved.