By potentiometric measurements using the electrochemical cell: Au, O-2, CO2\K2CO3(Au)\KBA(Au)\YSZ\KBA(Au)\(Borate glass), NiO, FeO\FeNi\Au the potassium activity of the heterogeneous phase system K-beta-Al2O3(KBA)/borate glass, MeO/Me has been determined between T = (350 and 670) degreesC. It is given by Ig a(K) = 4.590 +/- 0.42 - (17044.44 +/- 318.6K)/T. The establishment of the potassium chemical potential within the phase system results from the interaction between the borate glass and K-beta-Al2O3 yielding a defined content of potassium oxide that in its turn equilibrates with the oxygen potential coming from the interaction between Me and MeO. (C) 2004 Elsevier Ltd. All rights reserved.