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<-0.2V, Ge OH at U>0.1V in 0.5M H 2 SO 4 ). The surface reaction H a d OH a d is reversible and can be described by:(1)Ge H+p + +H 2 O 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<10ms) 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 >0.1V and longer times (t>10ms). The shift of flat band potentials yields a contribution Δφ D i p of OH dipoles to the total potential drop Δφ 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 θ. Reactions B and E are diffusion controlled, while surface reactions C and F are transfer controlled at low current densities. For θ H =θ O H =0.5, i.e.Δφ D i p =const., Tafel lines with b(C)=83 and b(F)=87mV are obtained. The correspondent transfer coefficients α(C)=0.7 and α(F)=0.3 indicate that they refer to different elementary reactions. Hence, an apparent equilibrium potential U 0 =0.01V, and apparent exchange current density i 0 =0.1μAcm - 2 can be estimated only from Tafel-plots.