The activation energies, E(a), for Ni2Si and NiSi formation were determined using in-situ resistance measurements with ramp rates ranging from 0.01 degrees C s(-1) to 100 degrees C s(-1). Measurements were performed using both conventional furnace and rapid thermal annealing. Ni films were evaporated on undoped polycrystalline Si and single-crystal Si on sapphire substrates. The E(a) values determined from Kissinger plots were 1.65 +/- 0.07 to 1.68 +/- 0.08 eV for Ni2Si formation and 1.84 +/- 0.05 to 1.87 +/- 0.06 eV for NiSi formation. These are the first reported measurements of E(a) values for Ni2Si and NiSi formation over such a wide range of heating rates (four orders of magnitude) and at such high heating rates. The phase-formation sequence remained the same for the range of heating rates examined.