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The distribution of vacancies throughout the underlying NiAs structure of pyrrhotite 5Cwas analysed through the application of vacancy avoidance and the closeness condition in conjunction with order–disorder layering. Two crystallographically equivalent structure solutions (chiral enantiomers) were produced consisting of layers containing one vacancy in every eight iron sites broken by a fully occupied layer every fifth iron layer, and best described by monoclinic statistical models. The statistical 5Cstructures were verified using synchrotron powder diffraction data as well as published electron-diffraction patterns. An order–disorder structure description is proposed for the intermediate pyrrhotites of which pyrrhotite 5Cis an end-member.