Findings concerning the origin of electron donor-acceptor complexes between olefinic hydrocarbons and chosen Lewis acids supplemented by new results are summarized. The important characteristics reported here are stoichiometric ratios of complex formation determined by the variation and titration methods, equilibrium constants and the thermodynamic parameters AG, AH and AS derived from them. Attention has been devoted to the characterization of frontier orbitals of olefins and Lewis acids using ionization potentials, half-wave polarographic oxidation potentials, electron affinities and half-wave polarographic reduction potentials. The aim consists in a quantitative characterization of redox processes which occur in elemental initiation reactions. The final evaluation of the data obtained has confirmed that the complexes involved in this case are weak and behave in accordance with the Mulliken theory.