The interface atomic structure of very thin IrSi(IrGe) films grown on the Ir(001) plane has been studied with the field ion microscope. Two distinctive types of structures have been observed. One shows theC(2×2) structure of the substrate. As the size of the layer is reduced by field evaporation, the surface relaxes into a rhombic structure resembling the (011) Ir layer of the IrSi(IrGe) crystal. The other shows a rectangular unit cell of a larger size, which is not yet successfully correlated to the structure of the IrSi(IrGe) crystal.