An ab initio MO study employing effective core potentials (ECP) on central atoms has been carried out for a series of hypervalent XH(5) hydrides, where X = P, As, Sb, and Bi. The XH(5)(D-3h) and XH(5)(C-4v) structures were found to be local minima and transition states (TSs) for Berry pseudorotation, respectively, with barriers of about 2 kcal/mol. All XH(5)(D-3h) are thermodynamically unstable with respect to XH(3)(C-3v) + H-2 by over 40 kcal/mol, and the periodic trend revealed in these stabilities is irregular. The energy decomposition analysis (EDA) was used to study the origin of this irregularity. The TSs for H-2 elimination from XH(5)(D-3h) possessed C-s symmetry and were equatorial-equatorial (eq-eq) for X = P, As, and Sb, whereas zwitterionic axial-equatorial (ax-eq) for X = Bi. The barrier heights to H-2 loss are relatively high and quite similar (30-34 kcal/mol) for all X, and all the XH(5) species are kinetically stable. The all-electron (AE) ab initio calculation was also performed for XH(5)(D-3h) and XH(5)(C-4v) with X = P, As, and Sb, showing that the effect of ECP and electron correlation on these structures was minor.