A modified Benedict–Webb–Rubin (MBWR) equation of state has been developed for Refrigerant 123 (2,2‐dichloro‐1,1,1‐trifluoroethane) based on recently measured thermodynamic property data and data available from the literature. Single‐phase pressure‐volume‐temperature (PVT), heat capacity, and sound speed data, as well as second virial, vapor pressure, and saturated liquid and saturated vapor density data, were used with multiproperty linear least squares fitting techniques to fit the 32 adjustable coefficients of the MBWR equation. Coefficients for the equation of state and for ancillary equations representing the vapor pressure saturated liquid and saturated vapor densities, and ideal gas heat capacity are given. While the measurements cover differing ranges of temperature and pressure, the MBWR formulation is applicable along the saturation line and in the liquid, vapor, and supercritical regions at temperatures from 166 to 500 K with pressures to 40 MPa and densities to 11.6 mol/L (1774 kg/m3). This formulation has been selected as an international standard based on an evaluation of the available equations of state by a group working under the auspices of the International Energy Agency.