In this Letter, we report that a new optical absorption around 470 nm (varying from 465 to 475 nm) occurs for the gold nanoparticles/monolithic mesoporous silica assembly, after exposure to ambient air (aging) and subsequent drying at 120 degreesC. Ambient ageing for a certain time plays a crucial role in appearance of this peak after subsequent drying at 120 degreesC. This new peak may be associated with Au clusters with size less than 1.5 nm. A nanodroplet formation and evaporation model is presented, which can qualitatively explain all evolution behaviours of this peak under different treatment conditions (C) 2003 Elsevier B.V. All rights reserved.