The acidity of europium(III)-exchanged zeolite L (Eu3+/ZL) and its influence on the luminescence performances of encapsulated europium(III) β-diketonate complexes are reported. The results indicate the importance of understanding the acidity of Eu3+/ZL in the preparation of luminescent host–guest materials of zeolite L crystals and europium(III) β-diketonate complexes. The acidity of Eu3+/ZL was determined roughly by using a dye (thionine) as the probe molecule. The luminescence performances of the host–guest materials, including excitation and emission spectra, lifetime of Eu3+ ions, quantum yield, and energy transfer efficiency, have been investigated. The luminescence behavior of the host–guest materials can be tuned by changing the acidity inside the channels of the zeolite L crystals and can potentially be used as a sensor for detecting ammonia.