Thin cobalt layers have been grown on and in porous silicon by chemical vapour infiltration and deposition using HCo(CO)(4). The process can be controlled to give a capping layer with or without metal in the pores. Thin coats on the pore walls have been measured to depths of several mu m and thick coats to depths of 0.1 mu m. Results from mass spectroscopy indicate that when a cobalt hydride precursor molecule decomposes on a pore wall it releases its hydrogen atom accompanied by three from the pore wall, so forming direct metal-silicon bonds.