A comparison is made between pure inorganic and hybrid organic/inorganic silica sol-gel systems, at different stages of evolution, using the information provided by the fluorescent probe 1,12-bis( 1-pyrenyl)-dodecane. The initial sols were prepared using tetraethylorthosilicate (TEOS) as precursor, with acid catalysis, at initial pH 1.2 and 2.5. The hybrid systems were prepared by incorporating a low molecular weight polymer in the initial mixtures (polytetrahydrofuran, M-w = 2000), in 1:100 molar ratio to TEOS. The evolution of the systems was followed by fluorescence spectra of the probe, incorporated in a very low concentration (similar to 10(-7) M). It has been shown that while in the pure silica systems the probe molecules are encapsulated in the primary silica particles as they form, in the hybrid systems most probes remain in the vicinity of the polymer, which has a strong hydrophobic character. The fluorescence decay curves of aged gels confirm that the initial pH mainly influences the dimensions of the primary silica particles that are much smaller at pH 1.2 than at pH 2.5. The role of oxygen as a fluorescence quencher has been analyzed, by comparing oxygen-free and air-equilibrated samples. The probe is accessible to oxygen throughout the sol-gel process for both systems at initial pH 2.5, while at pH 1.2 this is only true for the hybrid system.