Fluorine-bomb calorimetry was used to determine the massic energies of combustion of high-purity specimens of crystalline and vitreous germanium(IV) disulfide GeS2 according to the reaction : GeS2(cr or vit) + 8F(2)(g) = GeF4(g) + 2SF(6)(g). Standard massic energies of combustion at T = 298.15 K and p degrees = 101.325 kPa were found to be : Delta(c)u degrees(GeS2, cr) = -(25558.9 +/- 3.0) J.g-1 and Delta(c)u degrees(GeS2, 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 Delta(f)H(m) degrees(T = 298.15 K, p degrees = 101.325 kPa were derived : -(127.9 +/- 1.3) kJ.mol(-1) for GeS2(cr) and -(115.4 +/- 1.6) kJ.mol(-1) for GeS2(vit). This Delta(f)H(m) degrees(GeS2, 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 : GeS2(cr) = GeS(g) + 1/2S(2)(g) and that incorporate the new Delta(f)H(m) degrees(GeS,cr,298.15 K) obtained in Part II of this study (J. Chem. Thermodynamics 1994, 26, 727), yield several derived results for Delta(f)H(m) degrees(GeS2,cr,298.15 K) that are in reasonable agreement with the calorimetric determinations. The standard molar enthalpy of the transition : GeS2(vit) = GeS2(cr) : Delta(trs)H(m) degrees = -(12.5 +/- 1.5) kJ.mol(-1) at T = 298.15 K, calculated from the difference in the massic energies of combustion, is of about the same size as Delta(trs)H(m) degrees 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.)