Electrical measurements (resistivity, Hall effect, and superconducting critical temperature) are performed in epitaxial CoSi2 layers obtained by room‐temperature codeposition of Co and Si on 〈111〉 Si subsequently annealed between 250 and 650 °C. On the one hand, the CoSi2 layers annealed at low temperature (250–350 °C) exhibit poorer electrical characteristics than the films realized by solid phase epitaxy at 650 °C, because of both a lack of carriers and a degraded mobility. A possible origin of this fact could be the presence of unreacted Co atoms in the metal layer. On the other hand, the films annealed ex situ at 700 °C show excellent electrical characteristics, together with mirror‐like surfaces and extremely smooth Si/CoSi2 interfaces, for silicide thicknesses ranging from 35 up to 500 Å. Furthermore, by comparing the films obtained by the solid phase epitaxy and the codeposition techniques, we show that the long‐range roughness (few hundreds of angstroms) has no major influence on the steep increase of resistivity with decreasing film thicknes observed in ultrathin CoSi2 layers.