Pyrrhotites collected from sulfide concentrations in the irruptive and the underlying gneiss complex at the Strathcona Mine, Sudbury, were studied microscopically, by electron microprobe, thermomagnetic, X-ray, DTA and Moessbauer methods. The results show that two pyrrhotite types occur in this section through the orebody. The first is of composition (Fe,Ni) 7 S 8 with Ni [asymp] 0.35-0.50 wt. % and is ferrimagnetic up to 300-305 degrees C. X-ray powder data are in good agreement with those for "monoclinic" pyrrhotite with a 2A, 4C supercell. The second pyrrhotite is of composition (Fe,Ni) 9 S 10 with Ni [asymp] 0.68-1.01 wt. % and is ferrimagnetic between approximately 200 and 250 degrees C. X-ray powder data are in agreement with those for so-called "hexagonal" pyrrhotite with a 2A, 5C supercell. In the upper part of the section large amounts of both types of pyrrhotite occur intimately intergrown, but near the base of the irruptive there is an abrupt change to single-phase (Fe,Ni) 7 S 8 . Near the base of the orebody both types again become abundant. Electron microprobe analysis of the two pyrrhotite types and the associated pentlandite show greater concentrations of nickel in the pyrrhotites (of high susceptibility) and pentlandite towards the center of the section, with a concomitant decrease in the cobalt content of the pentlandite. The "hexagonal" pyrrhotite consistently shows an enrichment in residual nickel relative to the "monoclinic" type and is more readily etched by HI. Further studies of the two separate pyrrhotite types by Moessbauer spectroscopy and differential thermal analysis, together with the magnetic and X-ray data, suggest that each represents different, but definite, layer-order schemes of vacancies on the iron sites in the (001) planes. The Bertaut model for (Fe,Ni) 7 S 8 with the vacancies ordered in alternate iron atom planes parallel to (001), is consistent with these data and for (Fe,Ni) 9 S 10 a model with single and double filled planes alternating with those containing vacancies is proposed. Study of synthetic pyrrhotites containing low concentrations of nickel suggest that the nickel is evenly distributed over the magnetic sublattices in both types. This study suggests that variation in the original bulk composition of a sulfide melt is expressed as variation in the proportions of these essentially stoichiometric pyrrhotites and other co-existing sulfides.