A thermochemical analysis is performed in the system Cu–In–S at 298 K. Free energies of the compounds In6S7, In2.8S4, CuInS2, CuIn5S8, and of the recently discovered CuIn2 have been estimated, the numerical values (kJ/mol) of which are −1043 ± 21, −556 ± 8.8, −315 ± 54, −1238 ± 113, and −51 ± 26. A consistent set of data is used for the calculation of the Gibbs triangle as well as of the predominance area diagram. The results are in nearly complete agreement with the measurements published recently, in particular with those using the nuclear method of perturbed angular correlations (PAC). The compound CuInS2, one of the possible base materials for thin film solar cells, is shown to equilibrate with nearly all of the compounds of the system. A nonvariant four-phase equilibrium CuIn2−InS–In–CuInS2 is found at about 298 K. It is noted where more precise data are needed.