The development of stain etching relative to anodic etching for the production of porous silicon has been hampered by a lack of fundamental understanding. By applying Marcus theory we have arrived at a more expansive understanding of the charge transfer that initiates and controls etching. No credible information is available on the stoichiometry of stain etchants involving metal ion containing oxidants such as VO2+ and Fe3+. Herein we describe such experiments both for conventional stain etching and metal assisted stain etching in the presence of a pre-deposited catalyst such as Cu, Ag, Au, Pd or Pt.