The electrodeposition of thin films of the element cobalt on silicon wafers opens the possibility of producing magnetic structures on top of a technological substrate using an efficient and inexpensive method. In this paper, we present an extensive study of the electrodeposition of Co on Si(100) n-type substrates. Co thin films with metallic appearance and uniform thickness were obtained at deposition rates of 0.6 (2.8) nm/s for the 26 (104) mM Co electrolyte. A strong dependence of the surface roughness on the nucleation mechanism, i.e., instantaneous or progressive, was observed for layer thicknesses below 300 nm. Simultaneous magneto-optical Kerr effect and magnetoresistance measurements showed that magnetic behavior depends on the nucleation mechanism as well. Coercivity decays with increasing thickness, t, following a power law of the type t(-n), with n = 1.6 (2.1) for the 26 (104) mM electrolyte, which reflects a strong dependence of the magnetic behavior on surface effects. A large variation of the in-plane coercive fields was observed for the 104 mM electrolyte, which varied from 730 to 13 Oe as film thickness increased from 26 to 260 nm.