Enthalpies were determined for synthetic chalcopyrite (cufes2) between 298 deg. and 1,050 deg. K and for synthetic bornite (cu5fes4) between 298 deg. and 1,100 deg. K by copper-block drop calorimetry. Two transitions were found for each compound. The lower transitions were both crystal transformations from tetragonal (low-temperature form) to cubic (high-temperature form). The chalcopyrite transformation occurred at 830 deg. K with a heat of transformation of 2,405 cal/mole; the bornite transformation was at 485 deg. K with a heat of 1,430 cal/mole. The other transitions were of the second order and were presumed to be magnetic in character. The second-order transitions for chalcopyrite occurred at 930 deg. K, and the one for bornite at 540 deg. K.