The reflectivity of thin‐film platinum silicide was measured by means of a helium‐neon (He‐Ne) laser when samples of platinum films deposited on top of silicon wafers were annealed in a rapid thermal processor. This processor consists of two rows of tungsten‐halogen quartz lamps placed above and below a quartz processing chamber. The thermal cycles consisted of a fast heating (about 200 °C/s), followed by an isothermal plateau at temperatures ranging between 410 and 600 °C. Film reflectivities dropped in two stages, as a result of the reaction between platinum and silicon. This two‐stage drop was identified as due to the transformation of the platinum film, first into Pt2Si, and then into PtSi. The amounts of time required to complete the transformations were found to be in good agreement with the Arrhenius laws derived from the work of J. T. Pan and I. A. Blech [Thin Solid Films113, 129 (1984)] on isothermal low‐temperature (220–330 °C) sintering of platinum films on silicon, who unambiguously established the correlation between reflectivity changes and silicide formation.