The electrochemical oxidation-dissolution of (111) silicon has been investigated in HF-acetonitrile (MeCN) solutions. Four electrons per silicon were observed per oxidized silicon. Current doubling (quantum efficiency of 2.2) was observed for the photo-oxidation of n-Si. Unlike the etching of (111) silicon in aqueous HF solutions, no pores were formed in HF-MeCN. The etching of (111) silicon in HF-MeCN resulted in the formation of triangular pits defined by the (111) planes. A mechanism for the etching is proposed where steric hindrance of the surface terminated hydrogens induces bond strain and enhances the chemical reactivity. The mechanism also accounts for the formation of pores in (100) silicon, and the formation of a highly branched microporous structure when silicon is etched in an aqueous solution.