THF solutions of the new organometallic precursor Co(η³-C₈H₁₃)(η⁴-C₈H₁₂) react with 3 bar dihydrogen in the presence of polyvinylpyrrolidone (PVP) at three different temperatures, namely, 0, 20, and 60 °C to give colloidal solutions containing fcc cobalt particles of respectively ≤1 nm (Coll. 1) and ca. 1.5 nm (Coll. 2 and Coll. 3) mean sizes, as evidenced by HREM analysis. All colloids react with carbon monoxide to give new colloids (Coll. 1‘−3‘) displaying broad CO stretches near 2000 and 1890 cm^-1 on their infrared spectra recorded in KBr disks. The magnetic behavior of the colloids was studied by SQUID techniques. Coll. 1−3 display a typical superparamagnetic behavior with blocking temperatures in the range 9−10 K. From magnetization studies above and below the blocking temperature, it was possible to determine that Coll. 3 shows a mean size of 1.6 nm and a very narrow size distribution. Because of a magnetic surface effect, the magnetic moment per atom (μ_Co = 1.94 ± 0.04 μ_B) appears significantly larger than the value admitted for bulk cobalt. In contrast, Coll. 2‘ shows a weak magnetization and disappearance of the blocking temperature as a result of CO coordination.