The infrared absorption of Ir and IrSi thin films on Si substrates has been determined by transmission and reflection measurements over the wavelength range 2.5–25 &mgr;m. Detailed analysis of the dependence of absorption at 4 &mgr;m on film thickness indicates that a thin boundary layer with lower absorption than Ir is present at the Ir/Si interface and that such a layer with lower absorption than IrSi is present at the IrSi/Si interface. The existence of the boundary layers has been confirmed by the detection of oxygen at the interfaces by Auger analysis. Absorption and Auger measurements give no evidence of boundary layer formation at Pt/Si or PtSi/Si interfaces.