We present a photoemission study on reactivity of the Au-Si system. We studied gold films evaporated atop ultrathin silicon layers previously deposited on GaAs. Following analysis of both the Si 2p core level and the Au 5d valence band spectra related response, we show that the reaction mechanism between Au and Si is affected by structural imperfections of the silicon layer. This is in sharp opposition to some current models of the reactivity mechanism. (C) 2001 American Institute of Physics.