CO adsorption on NiSiO2 catalysts has been studied by two complementary techniques, infrared spectroscopy and saturation magnetization. Special care was taken to measure the degree of reduction and the metallic particle size. Two main bands attributed to CO bonded to Ni are observed: the A band at 2070-2040 cm−1, and the B band at 1935 cm−1 with a shoulder at 1800 cm−1. For completely reduced samples, the ratio of the integrated intensities of A and B bands, as well as the bond number n calculated from magnetic measurements (n = 1.85) are particle size independent in the 2.5–9.5 nm range, temperature independent in the 20–100 °C range, and coverage independent. On partially reduced samples, n is smaller and r higher. Experimental results are fully accounted for by assuming that the A band corresponds to a linear form and that the B band is a bridged species, the shoulder at 1800 cm−1 being attributed to multicentered species. Ni2+ or NiO present on partially reduced samples acts as a diluent of metallic nickel atoms, similar to Cu atoms in NiCu alloys; the stability of the concentration ratio of linear and bridged species suggests an analogy with coordination complexes. The presence of two bands in the region corresponding to linear species (2070 and 2040 cm−1) with intensities ratios varying with the degree of reduction, Ni particles size, coverage, and temperature is also discussed.