Abstract: Published data from pyrrhotine-vapour equilibrium experiments are used to make inferences concerning the high-temperature thermodynamic mixing properties of pyrrhotine. Darken's quadratic formalism provides the justification for plotting activity of sulphur data against (1−X)², where X is used in Fe_1−XS to express the composition of non-stoichiometric pyrrhotine. Such plots show that the thermodynamic behaviour of pyrrhotine appears to be different on either side of X = 0.125, the Fe₇S₈ composition.