An e.m.f. study of {xNa(2)O + (1 - x)P2O5}(1) containing x = 0.43 to 0.75 has been carried out over the temperature range 750 K to 1300 K. The set of thermodynamic properties, both partial and integral, is obtained. Significant negative deviations from ideal behaviour of Delta(f)G(m) and Delta(f)H(m) are observed. The results obtained are discussed in terms of the ideal-associated-mixture model. Computer-simulated and experimental results are in good agreement.