The accurate measurement of pH and potential in high-temperature high-pressure aqueous solutions using an external pressure balanced reference electrode (EPBRE) requires the development of accurate methods for calculating the thermal liquid junction potential (TLJP). Using well-developed irreversible thermodynamic techniques, as well as Agar’s hydrodynamic theory to calculate the ionic entropies of transport, we show that the TLJP makes a very significant contribution to the measured potential and can vary by more than +/- 150 mV depending on the temperature and composition of the non-isothermal electrolyte bridge. Our calculations of the TLJP are compared with experimentally measured data and good agreement is obtained. The development of an advanced EPBRE, in which the composition of the non-isothermal liquid junction is maintained accurately at a well-defined concentration, is considered to be an appropriate way to solve the problem of performing accurate potentiometric measurements in high-temperature high-pressure aqueous systems.