The gas-phase internal elimination (E-i) reaction of the sulfoxide (-SO-CH3) precursors of ethylene and model oligomers of PPV and PITN has been investigated by means of Hartree-Fock, Moller-Plesset (second and fourth order), and Density Functional Theory (B3LYP, MPW1K) calculations, Considerable differences between the obtained ground state and transition state geometries and the calculated activation energies are observed from one approach to the other, justifying first a careful calibration against the results of a benchmark CCSD(T) study of the El reaction leading to ethylene. In comparison with the CCSD(T) results, as well as with available experimental data, DFT calculations along with the MPW1K functional are found to be a very appropriate choice for describing the Ei pathway. The leading conformations of the precursors, the relevant transition state structures, and the energy barriers encountered along the lowest energy path to unsubstituted, alpha and beta chloro-, methoxy-, and cyano-substituted ethylene, styrene, stilbene in its cis and trans forms, and at last trans-biisothianaphthene have therefore been identified and characterized in detail employing DFT (MPW1K). Depending on the substituents attached to the C-alpha and C-beta atoms, different reaction mechanisms are observed.