We report on room temperature MeV Au ion induced modi. cations at the Co/Si interfaces. Nanometers size thin film of Co and Si were grown by ultra high vacuum (UHV) electron beam evaporation technique on Si(1 1 1) surface and were irradiated by 1.5 MeV Au2+ ions at a fluence of 5 x 10(14) ions cm (2). High-resolution transmission electron microscopy (HRTEM) along with energy filter imaging technique has been employed to study the formation of Co-Si alloy at the interface. Formation of such surface alloy has been discussed in the light of ion-matter interaction in nanometer scale regime. (C) 2009 Elsevier B.V. All rights reserved.