A study of trans-Rh(PH3)(2)(CO)X (X = F, Cl) using Hartree-Fock/many-body perturbation theory and density functional methods is presented. We report optimized Hartree-Fock and MBPT(2) structures employing effective core potentials with double-xi plus polarization basis sets including f-type functions. Optimized structures using density functional methods, including those with gradient-corrected exchange-correlation functionals, are also reported. Both ab initio methods yield complexes which have a slightly distorted square-planar structure with moderate bending of the phosphine ligands toward the halide. The predicted structure of the chloro complex is compared with experimental data for trans-Rh(PPh(3))(2)(CO)Cl.