Molecular aggregation and fluorescence processes of 9-aminoacridine (9AA) in thin silica gel films have been investigated by the steady state and time-resolved fluorescence measurements. The monomer of 9AA was the preferential species in the sol-gel reaction systems of tetraethylorthosilicate until the gelation occurred. The 9AA molecules formed the dimer or higher aggregates just after preparing the dip-coated thin film from the sol-gel system. The extent of the aggregation decreased in the film prepared from the system in which the reaction further proceeded. This result indicates that the aggregation in the prepared film was gradually prevented by the steric hindrance of the SiO(2) network with the progress of the sol-gel reaction. The fluorescence properties of 9AA revealed the behavior of the molecules due to the change in the physicochemical environment in the matrix. (C) 2010 Elsevier B.V. All rights reserved.