The homolytic bond dissociation enthalpies (BDEs) for the P-H bonds in substituted phenylphosphines (X-C6H4-PH2) have been estimated using the (RO)B3LYP procedure with the 6-311++G(2df,2p) basis set. The BDEs for the P-H bond in PH3, N-H bond in NH3 and X-C6H4-NH2 (X = H, CH3, and NO2), and the As-H bond in AsH3 have also been determined by the same computational procedure and compared with the experimental results to assess the reliability of our method. The proton affinities (PAs) of the arylphosphine anions (X-C6H4-PH-) have also been determined at the same level of theory. The effect of substituents on the BDE(P-H) and the PA values have been analyzed in terms of the substituent effects on the stability of the phenylphosphines, radicals, and anions. Additionally, accurate estimations for the electron affinity values of phenylphosphine radicals have been performed. We also explore the correlations of Hammett's substituent constants with the BDEs of the P-H bonds of phenylphosphines and the PA values of arylphosphine anions.