Dissolution characteristics for the photo-anodization of n-Si with three different doping levels (10(15)-10(19) cm(-3)) are studied. Three regions are observed : porous layer formation, illumination-limited current plateau and electropolishing. The limiting current is determined by the illumination intensity and the effective dissolution valence varies from 2 during porous silicon formation to 4 during electropolishing. Based on the experimental observations, a surface dissolution mechanism during porous silicon formation is proposed in which the hole capture is the rate determining step. The effect of doping level on the tunneling is discussed as tunneling increases with doping levels, and is not observed for low doping level (10(15) cm(-3)). The favorable conditions for porous silicon formation are discussed.