Cl-2/Cl- reference system was established with quartz cell where Cl-2 gas was trapped on a glassy carbon electrode surface allowing the thermodynamic equilibrium between Cl-2 and Cl- ions. Based on the Cl-2/Cl- reference, the equilibrium potential of Ag/AgCl reference electrode was evaluated at concentrations ranging from 0.00039 mole fraction AgCl to 1.0 mole fraction AgCl and temperatures from 723 K to 823 K. The measured values ranged from -0.877 V to -1.361 V and followed linear correlations against temperature and concentration. However, pure AgCl showed slight upward in comparison to other observed linear trends. Compared to the previously published data sets, this study provides a full spectrum of data yielding better understandings on Ag/AgCl reference, applicable for different experimental conditions. In addition, the activity coefficients of AgCl in LiCl-KCl salt were calculated using ideal Gibbs free energy at supercooled liquid standard state. The activity coefficients range from 0.0012 to 1.19, which did not conform to a linear relationship to AgCl concentration. (C) 2019 The Electrochemical Society.