The heat capacity of synthetic millerite, NiS, has been determined by adiabatic-shield calorimetry from 300 to 660 K where it transforms endothermally to the NiAs polytype (niccolite), Delta(trs)H(m) = 6591 +/- 50 J mol(-1). The measurements were continued to 1000 K. The latter polytype is easily quenched and its heat capacity has been measured from 260 K, through the semimetal-to-metal transition at 270 K with Delta(trs)H(m) = 1394 +/- 20 J mol(-1), and up to 340 K where it transforms exothermally to millerite with Delta(trs)H(m) (340 K) = -5774 +/- 50 J mol(-1). Thermodynamic function values for the NiS phases have been derived through integration of the present results and combining them with low-temperature results from the literature. The enthalpy increment for millerite at 298.15 K to the NiAs-type phase at 1000 K is 47140 J mol(-1), or about 7% higher than the drop-calorimetric results in the literature.