The IrSix contacts have been used in Accumulated Low Schottky Barrier MOSFET on SOI. An IrSix layer is formed as a result of reaction between metal and semiconductor during annealing. The process of silicidation in the Ir/Si/SiO2/Si structure has been studied by means of cross-sectional transmission electron microscopy (XTEM). The influence of annealing temperatures (300, 600, 900 ◦C) on silicide formation was analysed. The formation of a thin IrSix layer was observed at a temperature as low as 300 ◦C. In the Ir/Si/SiO2/Si structure annealed at 600 ◦C, the Ir atoms are shown to penetrate across the Si layer, causing disturbance of the Ir/Si interface. After annealing at 900 ◦C large IrSix islands were formed in Si. The expansion of the IrSix grains in the Si layer was observed. TEM results were correlated with XPS and electrical measurements.