The fundamentals of diabatic reactions are briefly reviewed. The two important types of radiationless transitions at the crossover region of two potential surfaces are considered kinetically and rate formulations are given. The first type involves an appreciable activation energy in excess of the heat of reaction. The second does not. The reaction velocity is formulated using the absolute rate theory where the transmission coefficient is interpreted in terms of the Landau–Zener probability approximation. The over-all rate of a sequence of reactions is treated in accordance with the principle that resistances in series are additive. If the second in a sequence of two reaction steps involves a negative activation energy and is slow because of failure to cross to the new surface the resultant repeated passage through the crossover region requires a special formulation of the transmission coefficient, which is given. Whether this transmission coefficient influences the over-all rate or not depends on whether or not it belongs to the rate-determining step.Quenching reactions are classified in terms of the number of free valences involved, an electron being excited from a saturated molecule conferring radical-like properties on the excited molecule. Excited odd-electron molecules will attract at considerable distances without appreciable activation energy.The transmission coefficient is discussed in terms of recruitment factors for cases in which the products of a reaction are out of equilibrium.Hot positive ions resulting from mass spectrographic reactions are indicated as another example of diabatic reactions, and a procedure for treating these by the activated complex theory is briefly mentioned.