Gas-phase densities of the systems : pure water, methane, il-hexane, Ii-octane, and benzene were measured at temperatures from 523.15 K to 673.15 K and at pressures up to approximately 100 MPa. Second and third virial coefficients were determined for each system, and the cross virial coefficients were calculated from earlier (p, rho, T, x) measurements. The uncertainties of the temperature and pressure measurements are within +/- 10 mK and +/-12 kPa, respectively, while the uncertainty of the density values is within +/-0.10 per cent. Parameters for the square-well and Kihara potentials have been re-evaluated from the new virial coefficients. Comparisons with values From independent sources have been made where possible.