A fundamental equation of state for the Helmholtz free energy of R 134a (1,1,1,2‐tetrafluoroethane) is presented which is valid for temperatures between 170 K and 455 K and pressures up to 70 MPa. It is based on the most accurate measurements of pressure‐density‐temperature (p,&rgr;,T), speed of sound, heat capacity, and vapor pressure which are currently available. A linear regression analysis and a nonlinear least squares fitting technique, based on the selected measurements, were used to determine the structure of the fundamental equation of state and the values of its 21 coefficients. The equation represents nearly all selected experimental data within their estimated accuracy with the exception of heat capacities and speed of sound in the region close to the critical point. Typical accuracies are ±0.05% for density, ±0.02% for the vapor pressure, or ±0.5 and ±1% for the heat capacity. This equation of state has been compared to equations established by other research groups by Annex 18 of the International Energy Agency (IEA) and has been selected as an international standard formulation for the thermodynamic properties of R 134a by this group.