Iridium silicides formation by rapid thermal annealing (RTA) under vacuum at several temperatures in the range of 350 to 650°C has been investigated. The substrates and the silicide films were analyzed by Rutherford backscattering spectrometry (RBS) and Auger electron spectroscopy (AES). At 350°C, no distinguishable phase was detected for 240 seconds of annealing time. At 400°C, for processing time up to 45 seconds only Ir1Si1 was formed, for longer processing time Ir1Si1.75 was formed too. At higher temperatures even for very short processing time, Ir1Si1.75 was formed. Ir, Ir1Si1 and Ir1Si1.75 were present simultaneously if the iridium film was thick enough and the processing time was long enough too. For thin iridium layers, the Ir1Si1 formed was totally converted to Ir1Si1.75, if the annealing time was long enough. Formation rates were observed to be three to five orders of magnitude faster than the reported for furnace annealing.