Abstract: In a method of preparing a luminescent aluminate comprising aluminium and one or more of the metals yttrium and elements of the lanthanum series the component metals of the aluminate are co-precipitated from aqueous solutions of salts of these metals by combining these salts with a precipitation agent comprising carbon dioxide and ammonia in a quantity from 1 to 3 mole per mole of carbon dioxide. Such a quantity of the precipitation agent is used that the molar ratio of the carbon in the precipitation agent to the sum of the metals in the salts has a value from 1 to 10, a higher value in that range being chosen for this ratio according as the desired average grain size of the precipitate is larger. Finally, the precipitate obtained is heated at an elevated temperature.