The self-consistent field (SCF) and B3LYP density functional methods in combination with the method of effective core potentials were used to calculate sructural and spectral characteristics of phosphorus trihalides and their adducts MX₃PX₃ (M = Al, Ga, In; X = F, Cl, Br and I). The calculated vibration frequencies of phosphorus trihalides are well consistent with published data. Halogen substitution for hydrogen in PH₃ leads to a significant weakening in donor properties and makes impossible existence of the phosphorus trihalide complexes in the vapor phase. The donor properties of PR₃ falls down in the series PBu₃ > PPh₃ > PMe₃ > PH₃ > PI₃ > PCl₃ > PBr₃ > PF₃. The dissociation enthalpies of MX₃PX₃ complexes with the same halide atoms vary in the series F > Cl > Br > I and Al > Ga < In. The experimental dissociation enthalpy of AlBr₃PBr₃ (92 kJ/mol) is probably erroneus.