Low-temperature heat capacities of cu2s(c) and cus(c) have been measured from 5 to 310 k with an adiabatic calorimeter. The results yielded s deg.298 = 27.76 For cu2s and s deg.298 = 16.05 For cus, where s deg.298 is the standard entropy at 298.15 K in cal k-1mol-1 (1 calth = 4.184 J). High-temperature relative enthalpies have also been measured with a copper-block calorimeter from 298 to 1,600 k for cu2s and from 298 to 780 k for cus. Two solid-solid transitions have been observed for cu2s upon heating, the first at 376 k with an isothermal heat absorption of 865 cal mol-1, and the second at 720 k with an isothermal absorption of 280 cal mol-1. The enthalpy of fusion at 1,400 k was 3,070 cal mol-1. No solid-solid phase transitions were observed for cus from 5 to 780 k. All related thermodynamic properties have been tabulated.