A new Helmholtz fundamental equation of state for 1,1,1,2-tetrafluoroethane was developed by considering its unified relation with intermolecular potential theory and its characteristics of the ideal curves. The equation is valid for temperatures from the triple point to 460 K, and pressures up to 70 MPa, and it behaves in a qualitatively rational manner even in the region far from the range of existing experimental data. The estimated uncertainties in the range of existing experimental data are 0.1% in density for the gaseous and liquid phases, 0.02% in the speed of sound for the gaseous phase, 0.5% in the speed of sound for the liquid phase, and 0.8% in isochoric specific heat for the liquid phase. The uncertainties of saturation properties are 0.05% in vapor pressure, and 0.25 and 0.5% in the densities of the saturated-liquid and saturated-vapor states, respectively. The equation of state has reliable third virial coefficients making it possible to derive reliable thermodynamic properties near saturation in the gaseous phase even at low temperatures that are useful to precisely design refrigeration and air-conditioning systems. (C) 2004 Elsevier B.V. All rights reserved.