Using the concept of resonant states, the expressions for the rate of assocn. and dissocn. are derived. A d. matrix formalism is developed, which permits discussion of the conditions for which conventional expressions of the reaction rates are valid. It is shown that an Arrhenius type expression is only valid if the widths of the resonant states are larger than pkT, which implies that the decay of resonant states is independent of the way they are formed. As long as this condition and the steady state assumption holds for the concn. of mols. in resonant states, the interference effects of overlapping resonances vanish. The connection with transition state theory is derived. General new expressions valid for all pressures are presented. The ...