The study of hydrogen evolution from temperature-programmed desorption (TPD) experiments appears to be so complex that it casts some doubt on the reliability of chemisorption techniques for the estimation of cobalt dispersion in a series of Co/SiO2 catalysts. Co particle sizes were examined by magnetic measurements (MMs), transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS). EXAFS shows that Co in the catalysts crystallizes in fee crystal lattice and that relatively small clusters are formed. The deduced average size is almost an order of magnitude smaller than that obtained by MMs and TEM, suggesting a complex morphology of Co particles. Study of size sensitivity in the Fischer-Tropsch reaction shows that intrinsic activity and chain growth probability first increase and then stabilize on increasing the particle size, thus providing a quantitative determination of the critical diameter separating these two zones, i.e., ca. 6 nm. Study of the surface species after reaction by TPH experiments indicates that (i) deactivation may be due to a loss of catalytically active cobalt; (ii) in the course of the reaction, neither formates nor acetates are detected in the catalyst; and (iii) four carbon-containing species are observed (alpha, beta, gamma, and delta). Species beta has been speculated to be the active phase of the reaction on the basis of its parallel variation with intrinsic activity. This intermediate, which is strongly interacting with the cobalt phase, does not contain oxygen atoms. The delta species might be associated with graphitic residue formation, whereas no specific role has been attributed to the gamma species.