The paper studies the influence of co-doping with alkali metal ions (Na+, Mg2+) on fast interconfigurational 5d-4f luminescence in LiSrPO4 doped with Pr3+. Na+ and Mg2+ ions were introduced in order to provide charge compensation and modify the electronic configuration of the materials. Luminescent properties of the powder samples were investigated using time-resolved pulsed cathodoluminescence and luminescence spectroscopy in the UV-VUV spectral region. The obtained results were compared with those obtained earlier on LiSrPO4:Pr3+ without co-dopants. With introduction of the co-dopants, slight changes in relative luminescence intensity and decay time of Pr3+ ion 4f15d1-4f2 radiative transitions were found. The study revealed a complex interplay between defects of crystalline structure and impurity ions leading to the peculiarities in energy relaxation processes. Energy transfer leading to the delayed recombination processes is responsible for significant afterglow in Pr3+4f15d1-4f2 emission. Display omitted •LiSrPO4:Pr3+,Na+/Mg2+ crystalline powders were synthesized via solid state reaction.•Emission and decay kinetics were studied upon UV-VUV and cathode ray excitation.•5d-4f Pr3+ emission shows fast decay (τ < 20 ns) with secondary slower components.•Defects of unknown nature negatively influence the relaxation processes.•Na+ and Mg2+ as co-dopants almost insignificantly improve luminescence properties.