Metal-assisted chemical etching (MaCE) of metallurgical-grade silicon (MG-Si) has improved the purity of MG-Si (∼99%) to close to solar-grade (∼99.9999%) by removing metal impurities during the successful preparation of porous silicon nanowires (SiNWs). A new etching principle is proposed to explain the different levels of chemical reduction between various metal impurities with pore formation during etching. This model provides chemical insights into the relationship between dissolved metal ions and pores evolved during the formation of SiNWs.