The technique of chemically enhanced vapour etching (CEVE) has been used in the fabrication of ultra-thin wires on Si substrates. Essential aspects of the CEVE method are first summarized. We then demonstrate the application of the technique in the selective patterning of covered with a monolayer of organic acid. Parallel studies of thin Co layers directly deposited on (111) and (100) Si substrates were employed to optimize the growth conditions. Conductance measurements of the thin films and patterned wires are briefly discussed.