The Na2O activity in the molten float glass composed of 13.2 m/o Na2O, 70.2 m/o SiO2, 10.0 m/o CaO, 5.9 m/o MgO, etc. was determined in the temperature range from 1260 to 1380 K by means of galvanic cells using Na-beta''-alumina as solid electrolyte. The Na2O activity, a(Na2O), accompanying the reaction Na2O (dissolved in Na2O-WO3 melt) = Na2O (dissolved in molten glass) was found to be given by log a(Na2O) = -5.88 - 6.32 x 10(3) T-1 as a function of temperature, and the relative partial molar enthalpy of Na2O was calculated to be DeltaH(Na2O) = -121.0 kJ/mol. (C) 2004 Elsevier B.V. All rights reserved.