The literature contains no reliable values for the entropy of molten sodium polysulphide, Na2Sx (3 less than or equal to x less than or equal to 5). In this paper we show how these values can be derived from a knowledge of the activity of sulphur in equilibrium with each polysulphide, as a function of temperature. The necessary information can be deduced from measured values of the emf, E, of the sodium-sulphur cell, and its temperature coefficient, (partial derivative E/partial derivative T)(p). The entropy values obtained at 600 K are : Na2S5.2 (in equilibrium with pure liquid sulphur), 446 J K-1 mol(-1); Na2S5, 434 J K-1 mol(-1); Na2S4, 372 J K-1 mol(-1); Na2S3, 308 J K-1 mol(-1).