The reaction of ethanediol with periodate ion has been investigated in detail at six temperatures, seven pH＇s, and multiple glycol concentrations. From these data, a quantitative determination of rate constants and activation energies as well as equilibrium constants and enthalpies for complex formation has been made as they vary with the applied conditions. The data have been interpreted as either first order with the complex serving as an intermediate or as second order with the complex serving as an inert entity. Additional equilibria that impact the reaction are hydration of I0(4)(-) in neutral conditions and the protonation of H5IO6 in acid solutions. In evaluating the data, the following generalization has been applied : Any condition that alters the ground-state energy of a reactant or reactants by an equilibrium process must in tom alter the activation energy by an equal amount. The conclusions that have been made are as follows : (1) I0(4)(-) is the primary reactant with ethanediol; H(5)I0(6) is a secondary reactant. (2) Characteristics of the previously identified complex are further defined. (3) There are complementary deviations in second-order activation energies versus enthalpies of complex decomposition as temperatures and pH are changed. The second-order activation energy plus corresponding enthalpy of complex decomposition equals the first-order activation energy which is invariant under all conditions. (4) There is no correlation between rate constants and complex formation. (5) Pinacol does not form an identifiable complex and reacts primarily with HSIO6, yet the variations in activation energies with pH are essentially the same for pinacol and second-order ethanediol. (6) With the evidence of similarity of second-order ethanediol rate data to pinacol data and the constancy of first-order ethanediol rate constants, it is concluded that the complex under consideration is not an intermediate, but instead deactivates the reactants. A second presumed complex or deviate from the observed complex is the true intermediate.