Bromination and oxymercuration of carbon-carbon double bonds represent classical examples of electrophilic addition, but they nonetheless show quite divergent patterns of reactivities for various olefins. However, the olefin reactivities are identical when the differences between the steric effects in the transition states for bromination and oxymercuration are explicitly taken into account. Steric terms are developed for olefin-bromine and olefin-mercury(II) interactions from the information included in the charge-transfer (CT) transition energies for the electron donor-acceptor (EDA) complexes. CT absorption spectra of various olefin-mercury(II) complexes are reported for the first time and are compared to the spectra of analogous EDA complexes with bromine. The striking similarity between the activation processes for bromination and oxymercuration, after inclusion of the steric term, derives from a unified mechanism for electrophilic addition to olefins.