This paper presents speed of sound measurements on pentafluoroethane (R125) in the vapor phase. The measurements were performed in a stainless steel spherical resonator of similar to 900 cm(3) at temperatures in the range 260-360 K and pressures up to 500 kPa. Acoustic virial coefficients and ideal gas heat capacities are deduced directly from the data. The whole set of speed of sound measurements and the ideal gas heat capacities are then correlated in the forms u(2)(T, p) and c(p)(o)(T), respectively. Analytical expressions for the temperature dependencies of the thermodynamic virial coefficients, based on a hard-core square-well potential, are then assumed and the model is fitted to the acoustic data, obtaining a virial equation of state for the vapor phase. A highly accurate Helmholtz equation of state a(rho, T) is established on the basis of the measured data, representing the (p rhoT) surface of the vapor phase in the same temperature and pressure ranges. The ideal gas Helmholtz equation a(o)(rho, T) is obtained from the former c(p)(o)(T) correlation. Given the high accuracy of the equation form, only very precise experimental data, such as acoustic measurements, are suitable for fitting the a(R)(rho, T) equation parameters. Both the thermodynamic models are validated on available density data. The good level of consistency reached by the Helmholtz equation, shows its form to be very reliable.