The gaseous PVTx properties of ethyl fluoride (HFC-161) + pentafluoroethane (HFC-125) mixtures were measured at temperatures from 305.154 to 393.228 K and corresponding pressures from 1032.5 to 5416.5 kPa using the isochoric method. The uncertainties in the present measurements 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 binary HFC-161/125 system using the well-known mixing rule for the virial coefficients. This equation reproduced the experimental data in the gas phase within +/- 0.210% in pressure and within +/- 0.313% in density. (c) 2005 Elsevier B.V. All rights reserved.