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-trifluoroethane (HFC-143a) was conducted in the range of temperatures from 308.138 to 403.205 K, pressures from 1077.3 to 4684.3 kPa, densities from 0.5144 to 2.0224 mol dm(-3), and compositions from 0.3852 to 0.8703 mole fractions of HFC-161. 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 HFC-161/143a system. This equation reproduced the experimental data in the gas phase within +/- 0.247% in pressure and within +/- 0.254% in density. (C) 2007 Elsevier B.V All rights reserved.