In this work is reported the sensitization effect by polymer matrices on the photoluminescence properties of diaquatris(thenoyltrifluoroacetonate)europium(III), [Eu(tta)₃(H₂O)₂], doped into poly-β-hydroxybutyrate (PHB) with doping percentage at 1, 3, 5, 7 and 10% (mass) in film form. TGA results indicated that the Eu³⁺ complex precursor was immobilized in the polymer matrix by the interaction between the Eu³⁺ complex and the oxygen atoms of the PHB polymer when the rare earth complex was incorporated in the polymeric host. The thermal behaviour of these luminescent systems is similar to that of the undoped polymer, however, the T_onset temperature of decomposition decreases with increase of the complex doping concentration. The emission spectra of the Eu³⁺ complex doped PHB films recorded at 298 K exhibited the five characteristic bands arising from the ⁵D₀ → ⁷F_J intraconfigurational transitions (J = 0–4). The fact that the quantum efficiencies η of the doped film increased significantly revealed that the polymer matrix acts as an efficient co-sensitizer for Eu³⁺ luminescent centres and therefore enhances the quantum efficiency of the emitter ⁵D₀ level. The luminescence intensity decreases, however, with increasing precursor concentration in the doped polymer to greater than 5% where a saturation effect is observed at this specific doping percentage, indicating that changes in the polymeric matrix improve the absorption property of the film, consequently quenching the luminescent effect.