Thin sol-gel films including rhodamine B (RE) which were dip-coated using the sol-gel reaction of tetraethyl orthosilicate have been prepared as a function of time after mixing of the reaction systems. The absorption and fluorescence spectra of the individual thin films have been observed as a function of time after the preparation of the individual thin films. The relative contribution of the monomer, H-dimer, and J-dimer of RE existing in the individual thin films to the total absorption and fluorescence spectra of the individual samples was obtained as a function of time. RE molecules were encapsulated in a certain structural region (prestructure of the pores) long before the gelation point. With the progress of the sol-gel reaction, dimerization in the prepared film was gradually prevented, and the monomer RE became the preferential species. On the other hand, dimerization of RE in the thin film was pronounced just after dip-coating until the SiO2 network was almost formed, and the dimerization was enhanced progressively with the time course of the reaction under a specific reaction condition. Water plays an important role in the dimerization process.