Adiabatic ionization potentials (AIPs) and vertical ionization potentials (VIPs) for all fluorinated, chlorinated, and chlorofluorinated ethylenes have been determined by ab initio computations. The calculated AIPs give a mean absolute deviation of 0.014 eV at G2 and 0.015 eV at G3 theories compared to experimental values. We have estimated AIPs (in eV) for AIP ((E)-CHClCFCl) = 9.59, AIP ((Z)-CHClCFCl) = 9.60, AIP (CCl2CFCl) = 9.42, and AIP (CHFCCl2) = 9.65. Furthermore, our calculated AIPs values of 9.58 eV for (Z)-CFClCFCl and 9.56 eV for (E)-CFClCFCl are very different from the experimental data of 10.2 eV. VIPs are calculated by Koopmans's theorem with HF methodology and by G2 and G3 theories. Koopmans's theorem fails in giving a good description of the behavior of the VIPs for fluoroethylenes. Furthermore, significant improvement in the results is observed by the mean absolute deviation from experimental data on the computed values (0.242 eV using the 6-311+G(3df,2p) basis set and 0.248 eV using the GTlarge basis set, compared with 0.049 eV at G2 and 0.045 eV at G3 theories) when orbital relaxation and changes in electronic correlation and zero-point energies are taken into account. Our estimated VIPs values calculated by G3 theory (in eV) are VIP ((E)-CHClCFCl) = 9.89, VIP ((Z)-CHClCFCl) = 9.90, VIP ((E)-CFClCHF)= 10.26, VIP ((Z)-CFClCHF) = 10.25, VIP ((E)-CFClCFCl) = 9.93, VIP ((Z)-CFClCFCl) = 9.96, VIP (CCl2CFCl) = 9.71, VIP (CF2CHCl) = 10.19, VIP (CHFCCl2) = 9.96, VIP (CH2CFCl) = 10.32, VIP ((Z)-CHClCHF) = 10.16, and VIP ((E)-CHClCHF) = 10.16. Furthermore, the variation of the VIPs and AIPs with the increase in the number of halogen atoms in the molecules presents different patterns to chloroethylenes and fluoroethylenes.