The PVT properties of 1,1,1-trifluoroethane (R-143a) in the gas phase have been measured by a Burnett apparatus in the range of temperatures from 320 to 380 K and at pressures up to 6 MPa. The vapor pressures in the range of temperatures from 295 to 342 K have also been measured, and a vapor-pressure correlation has been developed. The critical pressure was determined to be (3.776 +/- 0.005) MPa on the basis of the present measurements. The second virial coefficients have been determined along seven isotherms, and a truncated virial equation of state has also been developed to represent the temperature dependence of the second virial coefficients and the present Burnett PVT measurements. The experimental uncertainties of temperature, pressure, density, and second virial coefficient throughout the present study were estimated to be no more than +/-8 mK, +/-0.8 kPa, +/-0.12%, and +/-5%, respectively. The purity of the R-143a sample used was better than 99.95 mol%.