Co-passivated porous silicon (CPS) was prepared by stain etching. CPS samples prepared at different etching stages have different morphologies. All these morphologies are significantly different from those of conventional porous silicon (PS) etched in none cobalt-etching solution. The experimental results indicate that Co atoms only exist in a very thin layer on CPS surface, where Co atoms are well-distributed and Co atoms have hardly diffused into the substrate. Compared with the formation mechanism of conventional PS, there are two routes to generate holes in the formation of CPS, and there is NO2 gas evolved from etching solution in a certain condition during the etching. (C) 2004 Published by Elsevier B.V.