We have studied the dependence of the electrical and structural properties of ultrathin cobalt suicide films on the annealing temperature and deposited Co thickness. If less than 10 A of Co is deposited, epitaxial type B CoSi2forms immediately. As the deposited thickness approaches 10 A, small amounts of Co2Si are observed. If greater than 10 A of Co is deposited, epitaxial Co2Si forms at room temperature, which proceeds either via the reaction Co2Si —> CoSi —> CoSi2or via Co2Si —> CoSi2during annealing. In these thicker films our results suggest that the formation of type A CoSi2is correlated with the presence of Co2Si; the presence of CoSi as an intermediate phase is correlated with the occurrence of type B CoSi2. Both film thickness and reaction temperature strongly influence the electrical transport in these films such that very high resistivities are encountered when films either become very thin or are reacted at low temperatures. In the former case the size effect is responsible whereas in the latter the transport properties are dominated by extensive atomic-scale disorder.