The behavior of Si electrodes in aqueous alkaline media has been investigated by voltammetric and impedance techniques. The flatband potential has been estimated on the basis of Mott-Schottky plots and photocurrent onset as E-FB = -1.15 V (SCE) for n-Si and E-FB = -0.45 V(SCE) for p-Si. A dissolution/passivation mechanism is proposed, assuming: (1) irreversible electrochemical oxidation of H-terminated Si to a monohydroxylated surface species, which may undergo either (2) irreversible electrochemical oxidation to a dihydroxylated species responsible of 2D surface blocking or (3) chemical dissolution as SL(II) regenerating an H-terminated surface; furthermore, (4) the blocking species may be dissolved chemically as Si(IV); (5) a chemical reaction with water generating monohydroxilated Si (and H-2) is also considered. A kinetic model is developed for the case of n-Si, assuming Helmholtz-layer control. The model reproduces the essential experimental features of, both J-E curves and impedance diagrams in the active dissolution and passivation regions but is not expected to be realistic in the passive region where 3D surface blocking should be considered.