The thermodynamic properties of Na2O-SiO2 and Na2O-SiO2-CaO melts have been measured using the galvanic cell\(\begin{array}{*{20}c} {O_2 (g), (Na_2 O), Pt} \\ {Na_2 O - WO_3 liq} \\ \end{array} \left| \begin{gathered} Na^ + \hfill \\ \beta - alumina \hfill \\ \end{gathered} \right| \begin{array}{*{20}c} {Pt,(Na_2 O), O_2 (g)} \\ {Na_2 O - SiO_2 - CaO liq} \\ \end{array} \) Activities of Na2O were calculated from the reversible emf of the cell. This is possible because the activity of Na2O in the Na2O-WO3 liquid is known from previous work. Data for the binary Na2O-SiO2 system were obtained between 1000 and 1100 °C and for compositions ranging from 25 wt pct to 40 wt pct Na2O. At 1050 °C, Log\(a_{Na_2 O} \) varied from approximately 10.2 at 25 wt pct Na2O to approximately −8.3 at 40 wt pct Na2O, the dependence with respect to composition being nearly linear. The Gibbs-Duhem equation was used to calculate the activities of SiO2(s), and the integral mixing properties,GM, HM, andSM, were derived. At the di-silicate composition,GM = −83 kJ/mol,HM = −41 kJ mol andSM = 33 J/mol K at 1000 °C. (Standard states are pure, liquid Na2O and pure, solid tridymite.) The activity data are interpreted in terms of the polymeric nature of silicate melts. Activities of Na2O in the Na2O-CaO-SiO2 system were measured for the 25, 30 and 35 wt pct Na2O binary compositions with up to 10 wt pct CaO added. The addition of CaO caused an increase in the activity of Na2O at constant\(N_{Na_2 O} /N_{SiO_2 } \). The experimental data agree well with the behavior predicted by Richardson’s ternary mixing model.