High dose ion implantation was used to form cobalt silicide on porous Si containing different concentrations of light impurities (C and O). Samples were implanted with 100 keV Co+ ions to fluences of at room temperature, 350°C and 450°C, and then annealed (600°C for 60 min + 1000°C for 30 min). The formed silicide compounds were studied by Rutherford backscattering spectrometry (RBS), glancing incidence X-ray diffraction (GIXRD) and four point probe resistivity measurements. The quality and type of the formed silicide were found to depend on the original impurity level as well as on the implantation temperature and annealing. Best results were obtained at 350°C, where a nearly rectangular low resistivity layer was formed immediately after implantation. Annealing at 600°C improved the layer resistivity, but the higher temperature annealing destroyed the porous layer.