The vapor composition above KI, FeI₂, CdI₂, KI-FeI₂-Ag and KI-CdI₂-Ag systems were studied by the Knudsen effusion mass spectrometry. Temperature dependences of partial pressures of gaseous species above pure iodides were determined. Equilibrium constants and reaction enthalpies of compounds with the participation of formed complex iodides were obtained. Structures, molecular parameters and thermodynamic functions of gaseous KCdI₃, KFeI₃ and KAgI₂ were calculated by quantum chemistry methods. The obtained values of formation enthalpies (Δ_fH⁰ (298.15) for gaseous complex iodides KCdI₃, KFeI₃ and KAgI₂ are equal to −320.0±7.3 −218.0±6.6 and −174.5±12.0 kJ/mol, respectively.