In order to identify the underlying factors determining barrier heights when hydrogen atoms add to alkenes, we present a theoretical framework isolating the fundamental quantum-chemical properties involved and enabling evaluation of the relative influence of each property. This approach describes the control of these barriers and motivates a series of experimental measurements as a rigorous test. A two-state avoided curve crossing model provides the essential description. but only when multiple excited states are combined to yield a mixed state of dual covalent-ionic character. We show that variations in mixed-state energy drive the evolution in barrier heights, and that by selecting a set of test reactions with diverse energetic and overlap interactions, one may discover which of several excited states dominates this evolution. Results from the experimental test show conclusively that it is variation in the lowest ionic-state energy, and not variations in either singlet-triplet splitting or reaction enthalpy that drive barrier height evolution over die series of H + alkene addition reactions. Combining this result with our earlier results for H-atom abstraction reactions, we have demonstrated that barrier heights of essentially all radical-molecule reactions with electrophilic radicals are controlled by the excited ionic states formed by the transfer of an electron from the molecule to the radical.