Electrical and microstructural changes of coevaporated Ir5 0Si50 have been studied as a function of temperature from room temperature to 970 °C. In situ resistivity measurements, in situ annealing and transmission electron microscopy, Rutherford backscattering spectroscopy and Seeman-Bohlin glancing angle incidence x-ray diffraction were applied. In the as-deposited state the alloy film is amorphous and exhibits semiconducting behavior. A two-stage phase transformation occurs in the process of heating. The first transition from amorphous to crystalline IrSi is nucleation controlled. It is accompanied by an abrupt decrease in resistivity and was found to be dependent on the heating rate and chemical composition. The second crystalline-to-crystalline transformation is controlled by nonstoichiometry of the alloy film. An excess of Ir atoms leads to formation of Ir2Si at ∼640 °C while an excess of Si atoms leads to formation of IrSi3 at ∼710 °C. The second transition is independent of the heating rate. The formation of both Ir2Si and IrSi3 decreases the electrical resistivity. The isothermal transformation from amorphous to crystalline structure follows a sigmoidal function. The apparent activation energy of this process is about 1.32 eV. The kinetic behavior implies random nucleation and interface-controlled two-dimensional growth of the crystalline phase.