THF solutions of the new organometallic precursor Co(eta(3)-C8H13)(eta(4)-C8H12) react with 3 bar dihydrogen in the presence of polyvinylpyrrolidone (PVP) at three different temperatures, namely, 0, 20, and 60 degrees C to give colloidal solutions containing fee cobalt particles of respectively less than or equal to 1 nm (Cell, 1) and ca. 1.5 nm (Cell, 2 and Cell, 3) mean sizes, as evidenced by HREM analysis. All colloids react with carbon monoxide to give new colloids (Cell, 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. Cell, 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 Cell, 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 (nu(Co) = 1.94 +/- 0.04 mu(B)) appears significantly larger than the value admitted for bulk cobalt. In contrast, Cell, 2’ shows a weak magnetization and disappearance of the blocking temperature as a result of CO coordination.