The compositions of natural mackinawites, (Fe,Ni,Co,Cu)1+x S, from 14 different localities are presented. Heating experiments were performed and the temperature of observed breakdown to pyr-rhotite does not appear to be a function of cation proportions. Indeed, the "breakdown" is not isochemical and involves the addition of sulfur from the surrounding minerals and therefore, is not representative of the upper thermal stability of mackinawite. In most natural occurrences, mackinawite occurs with chalco-pyrite. Based on phase chemistry, this would appear to be a metastable assemblage. However, recent developments, particularly in the Cu—Fe—S system, suggest that at low temperatures several reactions may occur which could thereby permit the stable existence of this assemblage.