Density functional theory (DFT) has been used to study the solvolysis process of the organophosphorus compound, O,O-diethyl p-nitrophenyl thiophosphate (Parathion, PTH) with α-nucleophiles [hydroxylamine anion (NH₂O⁻), hydroperoxide (HOO⁻) and simple nucleophile methylthiolate (CH₃S⁻) in both gas and aqueous phases. Formation of a trigonal bipyramidal intermediate at the phosphorus center followed by elimination of leaving group is considered to be the major solvolyzed pathway through addition-elimination scheme. In this study, although there are two possible orientations for incoming nucleophiles with respect to the substrate, the present reaction mechanism is found to be independent of this relative orientation. The proposed addition-elimination mechanism has been first explored here. The results indicate that the α-effect is observed in presence of solvent. Free energy barriers for NH₂O⁻ and HOO⁻ are comparable and lower than that for the simple nulcleophile, CH₃S⁻. An important physical insight of this study is that there is a significant influence of the reaction medium on the nucleophilic reaction for solvolysis of PTH irrespective of the relative orientation of incoming nucleophile group.