Because of the tunability of properties demonstrated by organic inorganic hybrid materials, they have attracted considerable attention in recent years. However, despite luminescence being one of the major research topics involving hybrids, SiO2 poly(methyl methacrylate) (PMMA) systems have been poorly explored. In this paper, Eu3+:SiO2-PMMA samples were prepared with different Eu3+/Si molar ratios in order to determine Eu3+ local coordination, the potential interferences of these ions on synthesis reactions and the optimized lanthanide concentration for improved luminescence output. Careful analysis of FTIR spectra showed the presence of Eu3+ ions coordinated to carbonyl groups in the hybrid matrix, while TG results, allied to FTIR, indicate dopant influence on the mechanisms of formation of polymeric chains. Photoluminescence results showed that, at higher lanthanide contents, the position of the D-5(0) -> F-7(0) band and spectroscopic parameters are modified, a clear signal of the formation of lanthanide clusters, which results in concentration quenching of luminescence.