Structural phase transitions in bornite (Cu 5 FeS 4 ) are associated with distinctive thermal anomalies. We investigated variations in the phase-transition temperatures of bornite using differential scanning calorimetry (DSC). The composition of 48 samples of natural and synthetic low bornite was determined using an electron microprobe operating in WDS mode. Temperatures of the low-intermediate and intermediate-high transitions of these samples were then determined by performing heating and cooling DSC scans over the range 50-300 degrees C. Heating-run peaks of thermal anomalies associated with the low-intermediate transition varied over the ranges 197-207 degrees C in natural samples and 154-201 degrees C in synthetic samples of bornite. The thermal anomalies associated with the intermediate-high transition occur between 259 and 273 degrees C in natural samples, and between 239 and 271 degrees C in synthetic bornite. The marked variations in temperatures of the intermediate-high transition are linearly related to the Fe content and the Cu:Fe ratio. No such relationships are associated with the low-intermediate transition, implying the existence of fundamental differences in the mechanisms of the two transitions. On average, synthetic samples of bornite underwent the low-intermediate transition at lower temperatures than natural samples, this phenomenon being attributed to the possible presence of a larger population of quenched-in vacancies and the lower concentration of trace impurities in synthetic bornite.