Combining X-ray powder diffraction with fluorescence measurements is an efficient way to determine the optimum activator concentration of inorganic phosphors. La_1–xEu_xAlO₃ samples (perowskite structure), prepared by solid state reactions with 0.5<><5.0 at="" 1480="" k,="" were="" investigated="" by="" both="" analytical="" methods.="" the="" luminescence="" emission="" spectra="" show="" strong="">⁵D₀-⁷F₁ and ⁵D₀-⁷D₂ transitions and fluorescence intensity reaches a maximum at 1 mol% Eu³⁺. X-ray diffraction shows the formation of an EuAlO₃ phase at Eu³⁺ concentration above 1 mol%. Below 1 mol%, Eu³⁺ substitutes for La³⁺, as can be seen from the change in the lattice constant c.