The potential energy surfaces for the unimolecular ground state elimination reactions of dichloroethylene (DCE) and trichloroethylene (TCE) are studied with ab initio molecular orbital calculations. By the gradient optimization with the second-order Moller-Plesset perturbation (MP2) method and single point calculations with the fourth-order Moller-Plesset perturbation (MP4) and the quadratic single and double configuration interaction including a triple contribution [QCISD(T)], many molecular elimination channels including three- and four-center HCl, H-2, and Cl-2 elimination and H and Cl migration reactions are systematically examined. For cis- and trans-DCE, the three-center HCl elimination with subsequent chlorovinylidene rearrangement has the lowest overall barrier, whereas for 1,1-DCE for which the three-center path is not available, the four-center elimination has a rather low barrier. Another path starting with the rearrangement of DCE isomers to 1,2-dichloroethylidene followed by HCl elimination is not far in energy from these paths, complicating the overall mechanism of HCl elimination. The H-2 elimination from DCE isomers also can take either the three-center path or the 1,2-dichloroethylidene path. For TCE, though the overall barrier to produce HCl+dichloroacetylene is the lowest for the four-center HCl elimination pathway, the production of HCl itself is most easily accomplished by the three-center HCl elimination, and the easiest path for disappearance of TCE is its rearrangement via H or Cl migration to give the 1,2,2-trichloroethylidene intermediate. Thus the overall mechanism of HCl elimination reactions from TCE can also be complicated.