Theories of barrier height control in radical-molecule reactions must be tested against data spanning a wide range in reactivity, by a method for separating multiple, correlated terms in the theories. Here we present an analysis technique designed to reveal reactant properties controlling reactivity and rate constant measurements for an extensive series of reactions where that control is very much in doubt. The measurements were made with a new high-pressure flow experiment designed specifically to facilitate the study of multiple radicals. The derivative technique consists of graphically analyzing partial derivatives of modeled barrier heights, using measured barriers and reactant properties. We use this technique to uncover the governing parameters for hydrogen atom abstraction reactions, which are dominated by an essentially ionic excited state of the reactants. More generally, multiple excited states contribute to barrier formation. with different states dominating for different classes of reactions. The new experimental apparatus is a significantly more flexible (and much smaller) version of our original high-pressure flow system. In this case, we use hydrogen atoms as the attacking radical, enabling a study of hydrogen atom addition to alkenes, where reactivity may he controlled by ionic states, singlet-triplet splittings, reaction enthalpy, or a combination of these factors. By using hydrogen atoms, we eliminate potentially confounding influences on the ground state, and by selecting a series of alkenes and haloalkenes to systematically vary ionization potential, singlet-triplet splittings, and Jr-electron density, we lay the foundation for an extensive study of barrier height control for this reaction class. The data presented here include the first temperature-dependent measurements for 9 of the 13 reactions studied.