The activation energies, Ea’s, for CoSi and CoSi2 formation were determined using in situ resistance measurements during rapid thermal annealing. Co films were evaporated on undoped polycrystalline Si (poly‐Si) and single‐crystal Si on sapphire (SOS) substrates. The resistance was monitored for heating rates from 1 to 60 °C/s up to 900 °C. There was significant thermal lag between the samples and thermocouple embedded in the susceptor wafer for heating rates greater than 20 °C/s. The thermal lag was quantified by melting Au‐Si, Al‐Si, and Ag‐Si eutectics, and shown to be consistent with finite element modeling. The Ea’s determined from Kissinger plots for heating rates ≤20 °C/s were 2.09±0.11 and 2.03±0.08 eV for CoSi formation and 2.91±0.22 and 2.81±0.23 eV for CoSi2 formation, for Co/poly‐Si and Co/SOS samples, respectively. © 1995 American Institute of Physics.