A mechanism for the (photo)anodic dissolution of silicon in HF containing solutions is proposed, which explains the dependence of both the photocurrent quantum yield and the efficiency for hydrogen evolution on the flux of absorbed photons. The model assumes that the chemical oxidation of an Si(II) intermediate to an Si(IV) product, which is accompanied by the formation of a hydrogen molecule, is catalyzed by a mobile Si(I) dissolution intermediate. The surface chemistry, corresponding to the proposed mechanism of anodic dissolution, is discussed. Furthermore, it is shown that the mechanism may provide an explanation, based on chemical kinetics, for initiation of pores during anodic etching.