Highly correlated methods were used to obtain the optimized bond lengths and vibrational frequencies of ClF and ClF-. With convergent quantum mechanical methods, the anion is much more difficult to treat than neutral ClF. Adiabatic electron affinities (EA(ad)), vertical electron affinities, and vertical detachment energies have been evaluated and compared to the controversial experimental values reported in the literature. Our best prediction for the zero-point vibrationally corrected EA(ad) is 2.25+/-0.1 eV. (C) 2003 American Institute of Physics.