The structures, vibrational frequencies, and thermodynamic stabilities of the homonuclear polyhalogen ions, X-3(+), X-3(-), X-4(+), X-4(-), X-5(+), and X-5(-) (X = Cl, Br, 1), have been calculated at the CCSD(T) level. The energetics were calculated using the Feller-Peterson-Dixon approach for the prediction of reliable enthalpies of formation. The calculations allow the following predictions where stabilities are defined in terms of thermodynamic quantities. (1) The X-3(+) cations are stable toward loss of X-2 (2) the X-3(-) anions are marginally stable toward loss of X-2 with Cl-3(-) being the least stable; (3) the X-4(+) cations and X-4(-) anions are only weakly bound dimers of X-2(+1/2) and X-2(-1/2) units, respectively, but the cations are marginally stable toward decomposition to X-3(+) and X, with I-4(+) having the lowest dissociation energy, whereas the X-4(-) anions decompose spontaneously to X-3(-) and X; (4) the X-5(+) cations are marginally stable at low temperatures toward loss of X-2, with Cl-5(+) being the least stable; and (5) the X-5(-) anions are also only stable at low temperatures toward loss of X-2, with Cl-5(-) being the least stable.