Porous silicon is usually formed under anodic polarization in an electrochemical cell. In this paper, a technique is described for forming porous silicon without using an external current or voltage source. By connecting an inert metal electrode to a silicon sample, both immersed in a HF solution, a galvanic cell is formed. Reduction of oxygen at the inert electrode results in the etching of Si at the silicon/electrolyte interface. Porous silicon is formed at a rate which is dependent on the cell current. The formation rate may be enhanced by adding oxidizing agents to the solution. Galvanic etching for forming porous Si is a promising alternative for stain etching since it gives more uniform and reproducible results.