The changes in surface topography of thin metallic films during ion bombardment were investigated with STM. Pd and Au thin films, having defect-free, atomically flat areas over at least 300×300Å2 were bombarded with Ar+, Kr+, Xe+ ions. Ion energies and target temperatures were varied. The roughening after high fluence irradiation shows scaling with a roughness exponent 0.84. For 300 K target temperature we observe, both for Au and Pd, 3 D erosion, for 700 K, on the contrary, layer-by-layer erosion. The differences in erosion morphologies between Au and Pd surfaces are supposed to stem from differences in surface mobility of adatoms.