Different surface states (Ge-OH and Ge-H) were found on n-Germanium. These two surface states can be controlled by potential (Ge-H at U < -0.2 V, Ge-OH at U > 0.1 V in 0.5 M H2SO4). The surface reaction H-ad <----> OHad is reversible and can be described by:Ge-H + p(+) + H2O <----> Ge-OH + 2H(+) + e(-)Measurements were carried out on n-Ge(111). Starting from Ge-H, anodic potentiostatic pulse experiments yield the kinetics of the surface reaction (1). Fast surface states, e.g. radicals were formed at relatively short times (t < 10 ms) due to the breaking of Ge-H-bonds. Then, OH-groups were generated and a Ge-OH monolayer was formed at polarization potentials U-p > 0.1 V and longer times (t > 10 ms). The shift of flat band potentials yields a contribution Deltaphi(Dip) of OH dipoles to the total potential drop Deltaphi at the interface. Reaction (1) was also studied by cathodic pulse experiments starting from Ge-OH.As a result, the kinetics of the surface reaction were obtained in dependence on U and theta. Reactions B and E are diffusion controlled, while surface reactions C and F are transfer controlled at low current densities. For theta(H) = theta(OH) = 0.5, i.e.Deltaphi(Dip) = const., Tafel lines with b(C) = 83 and b(F) == 87 mV are obtained. The correspondent transfer coefficients alpha(C) = 0.7 and alpha(F) = 0.3 indicate that they refer to different elementary reactions. Hence, an apparent equilibrium potential U-o = 0.01 V, and apparent exchange current density i(o) = 0.1 muA cm(-2) can be estimated only from Tafel-plots. (C) 2003 Elsevier Ltd. All rights reserved.