The electrochemical oxidation and passivation of Si(100) and Si(111) electrodes in KOH solution was studied by potentiodynamic and potential-step measurements. Striking differences were observed between the surfaces. A comparison of the results for n- and p-type electrodes led us to conclude that electrochemical oxidation of silicon in alkaline solution must be triggered by a chemical reaction. The strong influence of temperature on the current-potential and current-time results of (111) surfaces supports the importance of chemical activation. Photocurrent experiments on n-type (111) electrodes show that oxide nucleation is important for growth of the passive layer. A mechanism combining surface chemistry and electrochemistry is proposed to account for the pronounced anisotropy in anodic oxidation.