The thermodynamic properties of R134a and R123 are formulated using a modified Benedict-Webb-Rubin (MBWR) equation of state fit to experimental measurements of the critical point, vapor pressure, saturated liquid and vapor volumes, superheated pressure-volumetemperature (p-V-T) behavior, and second virial coefficients derived from p-V-T and sound speed measurements. The heat capacity of the ideal gas reference state is determined from sound speed measurements on the low density vapor. Surface tensions are also presented. The experimental methods and results are summarized, compared to the property formulation and, where possible, compared to other sources in the fiterature. Tables and diagrams of the thermodynamic properties of R134a and R123, prepared using the MBWR equation of state, are presented. While the various measurements cover different ranges of temperature and pressure, the MBWR formulation is applicable in both the liquid and vapor phases at pressures up to 10,000 kPa (1500 psia); the applicable temperature range is 233 to 450 K (-40° to 350°F) for R134a and 255 to 450 K (0° to 350°F) for R123.