For calculation of the thermodynamic properties of refrigerants it is proposed that the extended corresponding states principle (ECSP) should be used using two specific reference fluids, CFC12 (CCl2F2) and HFC134a (CF3CH2F), in order to cover the field of interest for most refrigerants. The specific parameters of these refrigerants have been determined for the Lee-Kesler modification of the Benedict-Webb-Rubin equation of state. In the parameter estimation, it has been taken into account that all variables are subject to errors by using the so-called Error-in-Variables-Model (EVM). The parameters have been estimated from experimental pressure-volume-temperature data, vapour pressure and saturated liquid and vapour volume data for CFC12 and HFC134a. The extended corresponding states principle proposed in this work with CFC12 and HFC134a as reference fluids has been used in calculations of the thermodynamic properties of a number of refrigerants. In comparison with the original Lee-Kesler equation this concept gives improved results in calculations of vapour pressure and liquid volume.