Singly and doubly excited configuration interaction calculations for the ground states of positronium halides, PsF, PsCl, PsBr, and PsI, are performed, and positron ionization energies, positronium binding energies, and two-photon annihilation rates are reported. It is shown that the correlation effects increase their values calculated by the Hartree-Fock method. In particular, the increases of the two-photon annihilation rates are very large. (C) 2003 Elsevier B.V. All rights reserved.