Relative enthalpies of cuprous iodide (cui) were measured at the Bureau of Mines to provide thermodynamic data for the advancement of mineral technology. Enthalpy measurements between 298.15 And 1,396.3 K were made with a copper-block drop calorimeter. The temperatures of transition were measured by differential scanning calorimetry. Reversible transitions were determined at 643 k with a standard enthalpy of transition = 0.74 Kcal/mol, at 679 k with a standard enthalpy of transition = 0.65 Kcal/mol, and at 868 k with a standard enthalpy of fusion = 1.895 Kcal/mol. Tabulated values are listed at selected temperature increments from 298.15 to 1,600 k for the standard relative enthalpy, heat capacity, entropy, and Gibbs energy function. The enthalpy data are also given in the form of equations and combined with thermodynamic data from other investigations to derive values of standard enthalpies of formation, Gibbs energies of formation, and logarithms of the equillibrium constants of formation (log kf).