The activation behaviors of air-passivated commercially pure Ti, Ti-50 wt % Zr, and Ti-45 wt % Nb were investigated in hydrochloric acid (HCl) ranging from 0.1 to 10 M, and temperatures ranging from 13 to 52 degrees C. in situ electrochemical impedance spectroscopy and X-ray adsorption studies in 5 M HCl confirmed: (i) oxide dissolution occurred uniformly and (ii) rapid drops in open-circuit potential (OCP) corresponded with the exposure of the underlying metal to solution. Activation energies varied from 56.5 +/- 8, and 62.9 +/- 5, to 103 +/- 6.2 kJ/mol for Ti, Ti-45% Nb, and Ti-50% Zr, respectively. Activation energies, low dissolution rates (<10-7 mol/m(2) s or 10(-3) nm/s) and dependency of the log of the oxide dissolution rate on the log of hydrogen ion activity ranging from 0.4-0.6 suggest that cleavage of metal cation-oxide bonds occurs by a rate-determining ion transfer step involving protonation of O-2 in oxides by one H+ ion to form free OH-.