The mixture of pentafluoroethane (R-125) and 1,1,1-trifluoroethane (R-143a) is a promising alternative for the conventional refrigerant R-502, In this paper, we present 220 PVTx property measurements of this binary system along four isopleths in the gas phase which were performed by an established Burnett apparatus. The uncertainties in the present measurements were estimated to be within +/-8 mK for temperature, +/-0.8 kPa for pressure, +/-0.15% for density, and +/-0.001 mole fraction for composition, respectively. On the basis of the present PVTx property measurements, which lie in the range of temperatures from 305 K to 380 K, pressures up to 4.8 MPa, densities from 0.04 mol.dm(-3) to 2.5 mol.dm(-3), and compositions from 0.27 to 0.74 mole fraction of R-125, a truncated virial equations of state with an accuracy of +/-0.2% in pressure was developed for the binary R-125/143a system using the well-known mixing rule for the virial coefficients : The temperature dependence of the second virial coefficients, including the cross-second virial coefficients for this system, is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.