A general discussion of several methods of SCF MO calculations, in particular MNDO, is given. The method by which transition states are located and tested is outlined. The structures and stabilities of a series of pentadienyl anions are studied, and polarization is identified as the most important stabilizing factor. Hyperconjugation is discussed on the basis of calculated and experimental ionization potentials of chloro-olefins. Reaction mechanisms are proposed for t-wo reactions of carbon atoms which were studied theoretically and experimentally. Calculated (MNDO) transition states for the hydrogen abstraction from ethane by methyl radical were located and led to good estimates of deuterium isotope effects.