Experimental observations are presented which indicate that porous silicon can be selectively converted to silicon dioxide. By presenting cross-sectional transmission electron micrographs, nuclear analysis data and electrical data, it is demonstrated that the selective oxidation occurs when the surface of the porous silicon has been amorphized by ion implantation. Electrical results are presented which show that the oxide is of sufficient quality for isolation purposes. It is of note that the partially oxidized porous silicon underlying the surface oxide layer, which initially was constituted of very thin, vertical, silicon 'wires', still exhibits a relatively low resistivity. The significance of these observations for device fabrication is briefly discussed.