The Ni-I species generated by UV-irradiation in the presence of hydrogen at 77 K of Ni/SiO2 samples where nickel was deposited on silica using four different methods are studied by electron paramagnetic resonance. Four different signals are observed and tentatively assigned to silica-supported Ni-I(H-2)(n) complexes resulting from the reduction of two different three-coordinated Ni-II precursors (Ni-3c(II)). The total amount of Ni-I obtained by photoreduction was measured using a new titration method based on CO chemisorption. The principles of this method are described. The reduction degree is related to the amount of Ni-3c(II) precursors. The highest reduction degree ([Ni-I/Ni-total] = 40%) is obtained for samples where Ni2+ was introduced by cationic exchange of the silica support with [Ni(en)(3)](2+) (en = ethylenediamine). The results are consistent with concomitant electron paramagnetic resonance measurements.