We demonstrate that spa water (Tenkeisen Spa water; TeSW) can be used as a solution for preparing porous silicon (PSi). The TeSW is a colorless alkaline solution. Photoetching is carried out on an n-type Si(111) wafer in TeSW under 5 mW He-Ne laser illumination. Photoluminescence from PSi shows a peak at similar to 1.95 eV at room temperature. The structural and optical properties of the TeSW-prepared PSi sample are nearly the same as those prepared in HF solution, although neither H+ nor F- is present in the spa water. This is the first report of preparing light-emitting PSi using alkaline solution. (c) 2005 The Electrochemical Society.