Ab initio molecular orbital calculations on the ion-molecule reactions of the tripler P+ with ClH and FH have been performed at the G1*, G2* (G1 and G2 methodologies with MP2/6-31G(d,p) base geometries), MCSCF, and QCISD(T) levels of theory. Two reaction mechanisms leading to PX(+) + H (X = Cl, F) products have been investigated. Both of them proceed through the formation of an ion-molecule complex, in the first step of the reaction, without energy barrier. Reaction pathway I involves the abstraction of hydrogen from the X atom. A significant configuration mixing is found to take place for this abstaction, and we have found that it is energetically unfavored with respect to the reactants. Reaction pathway II consists of the transposition of the hydrogen from the X atom to P, to form the HPX(+) intermediate, and then abstraction of the hydrogen from P. Reaction pathway II, which is energetically more favorable than pathway I, could be a likely source of P-Cl (X = F, Cl) bond containing compounds in the interstellar medial however, it is predicted to be an unlikely source for P-F compounds.