The enthalpies of formation of organophosphorus(III) compounds have been calculated at the G3X, G3X-(MP2), and B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G(d, p) levels of theory using the atomization energy procedure and the method of isodesmic reactions. The Delta H-f degrees(298) values for 50 relatively large molecules with up to 10 non-hydrogen atoms, such as P(CH3)(3), P(C2H5)(3), P(OCH3)(3), n-C4H9OPCl2, [(CH3)(2)N](2)PCl, (C2H5)(2)-NPCl2, and [(CH3)(2)N](2)PCN, have been calculated directly from the G3X atomization energies. A good agreement between the known experimental values and G3X results for 14 compounds provides support to our predictions for remaining species whose experimental enthalpies of formation are unknown or known with relatively large uncertainties. On the basis of our calculations and sometimes conflicting experimental data a set of internally consistent enthalpies of formation has been recommended for organophosphorus(III) compounds. Our computational results call into question the experimental enthalpies of formation of P(C2H5)(3) and P(n-C4H9)(3). From comparison with most reliable experimental data, the accuracy of the theoretical enthalpies of formation is estimated as ranging from 5 to 10 kJ/mol. The recommended Delta H-f degrees(298) values were used to derive the group additivity values (GAVs) for 45 groups involving the phosphorus(III) atom. These GAVs significantly extend the applicability of Benson's group additivity method and may be used to estimate the enthalpies of formation of larger organophosphorus(III) compounds, where high level quantum chemical calculations are impracticable.