In situ resistivity measurements have been utilized to study the reaction and silicide formation between cobalt and amorphous silicon thin films from room temperature to 800 °C. In conjunction, structure and composition changes were analyzed by x-ray diffraction and Rutherford backscattering spectrometry. Formation of Co2Si, CoSi, and CoSi2 were observed. Interfacial reaction to form Co2Si occurs at approximately 400 °C. In bilayers of excess silicon, CoSi forms at approximately 520 °C and, if free silicon is still present, CoSi2 forms at about 550 °C. In the case of excess cobalt, Co2Si forms first and is followed by a cobalt-rich solid solution. Co3Si silicide was not observed.