It is experimentally demonstrated that, after ion irradiating 60 nm thick Au films on Si substrates with 230 keV Ar+ ions, annealing conditions can be found leading to strong diffusional contrasts between bombarded and unbombarded areas. While Au readily diffuses into the bombarded part of the sample at 310 °C, its diffusion is still completely blocked under identical conditions in the unbombarded parts. Clear evidence is provided that this diffusional contrast is due to bombardment induced amorphization of the underlying Si substrate. The amorphous Silicon (a-Si), however, has to extend right to the Au/Si interface, since any intermediate crystalline layer will suppress the diffusional contrast. An example for this latter situation is realized by performing the ion bombardment prior to the evaporation of the top Au layer leading to a still crystalline Si surface layer, which is found to act as a barrier against Au diffusion at 310 °C. In accordance with the idea that a-Si, independent of its specific preparation, causes the observed Au diffusion enhancement, the effect is also found for a-Si prepared by evaporation at ambient temperature. In that case an even higher Au diffusion coefficient is obtained than for Si amorphized by ion bombardment pointing to subtle structural differences between both types of amorphous Si.