The adsorption kinetics of hydrogen and hydroxyl on Pt(111) and stepped Pt[n(111) × (110)] electrodes with n = 29, 9, and 4 in acidic and alkaline electrolytes have been studied using impedance spectroscopy. We found a potential dependent charge transfer resistance, R(ct), (∼30 Ω cm(2) to ∼1 kΩ cm(2)) for hydrogen underpotential deposition in alkaline media (0.05 M NaOH), whereas in acidic media (0.025 M HClO(4)) R(ct) was too low to be determined accurately. Assuming simple (mean field) isotherms to fit our data, we obtain repulsive interactions between H(upd) at the (111) terrace and effective attractive interactions at the steps. The adsorption of OH on (111) terraces is fast in both acidic and alkaline media.