The growth kinetics of the iridium silicides, IrSi and IrSi1.75 formed by RTA in vacuum has been characterized as a function of the processing temperature, up to 675°C, and annealing time for iridium films of different thickness. The sample temperature was measured using as a reference the SPE growth rate of amorphized silicon processed in the same RTA conditions. The samples were characterized by RBS, HREM, XRD and microdiffraction. RBS spectra show that the transition region between IrSi and IrSi1.75 is slightly gradual, while the interface between the IrSi and the iridium metal is sharp. The layer thickness of each silicide as a function of temperature and time was obtained from the RBS spectra. The activation energy of the diffusion coeficient of Si in IrSi and IrSi1.75 are about 1.3 eV and 3.9 eV respectively. Phase kinetics are the same for both thin and thick samples processed at the same temperature and annealing time when unreacted iridium remains in the film. However, the phase evolution is different when the annealing time is long enough to convert all the iridium metal into silicide in the thin film sample but not in the thick one. In this case, IrSil.75 is the only phase found if the annealing time is long enough. Silicides formed at 525°C consist mainly of polycrystalline IrSi, with grain size between 3 and 10 nm, and some isolated grains of IrSi 1.75. The interface between the IrSi and the Si is flat, very sharp, and well oriented grains grow from it. In order to avoid the IrSi1 75 formation and to obtain IrSi layers at reasonable rates, an RTA process at 500°C is proposed.