Fluorine-bomb calorimetry was used to determine the massic energies of combustion of high-purity specimens of crystalline and vitreous germanium(IV) disulfide GeS 2according to the reaction: GeS 2(cr or vit) + 8F 2(g) = GeF 4(g) + 2SF 6(g). Standard massic energies of the combustion at T= 298.15 K and p° = 101.325 kPa were found to be: Δ c u°(GeS 2, cr) = −(25558.9 ± 3.0) J·g −1and Δ c u°(GeS 2, vit) = −(25650.2 ± 4.5) J·g −1, where the uncertainties are standard deviations of the mean of the individual experimental results. Standard molar enthalpies of formation Δ f H m°( T= 298.15 K, p° = 101.325 kPa) were derived: −(127.9 ± 1.3) kJ·mol −1for GeS 2(cr) and -(115.4 ± 1.6) kJ·mol −1for GeS 2(vit). This Δ f H m°(GeS 2, cr) is in close agreement with the result from a previous determination ( J. Chem. Thermodynamics 1984, 16, 335), also by fluorine-combustion calorimetry, and a weighted mean of the two:−(127.8 ± 1.0) kJ·mol −1, is recommended as a “best” value. Third-law treatments of equilibrium vapor pressures as a function of temperature reported in the literature for the high-temperature decomposition reaction: GeS 2(cr) = GeS(g) + 1 2 S 2(g) and that incorporate the new Δ f H m°(GeS, cr, 298.15 K) obtained in Part II of this study ( J. Chem. Thermodynamics 1994, 26, 727), yield several derived results for Δ f H m°(GeS 2, cr, 298.15 K) that are in reasonable agreement with the calorimetric determinations. The standard molar enthalpy of the transition: GeS 2(vit) = GeS 2(cr): Δ trs H m° = −(12.5 ± 1.5)kJ·mol −1at T= 298.15 K, calculated from the difference in the massic energies of combustion, is about the same size as Δ trs H m° for similar compounds of the Group-14 elements. (Throughout this abstract, uncertainties in all molar quantities are expressed as twice the standard deviation of the mean.)