Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each of the cationic structures of FOCl+ and FClO+ at various levels of theory, employing second-order Moller-Plesset perturbation interaction theory (MP2), singles and doubles excitation configuration interaction theory (CISD), and coupled-cluster theory with perturbative triple excitations [CCSD(T)]. Of the ions, FClO+ has the lowest energy structure, with an energy separation of 25.7 kcal mol(-1). The adiabatic ionization potential for HOCl is used to calibrate the values for FOCI and FCIO. From the calibration the probable uncertainty in the adiabatic potentials for the formation of FClO+ and FOCl+ is estimated. The adiabatic ionization potentials for the formation of FClO+ and FOCl+ are estimated as 246.9+/-3 kcal mol(-1) and 267.7+/-3 kcal mol(-1), respectively.