Abstract: Method of preparing a luminescent Eu.sup.2+ activated strontium aluminate defined by the formula Sr.sub.4 Al.sub.14 O.sub.25 :Eu.sup.2+ in which per mol of the Al present a combination of between 0.1 and 5 mol % of B.sub.2 O.sub.3 or compounds yielding B.sub.2 O.sub.3 upon an increase of temperature, and between 0.1 and 10 mol % of at least one of the fluorides and/or chlorides of Ba, Sr, Cs and/or Al and/or RbCl as a flux to a mixture comprising SrO, Al.sub.2 O.sub.3 and Eu.sub.2 O.sub.3 or compounds yielding these oxides upon an increase of temperature, whereafter the mixture is subjected to a first heat treatment in a weekly reducing atmosphere at 1100.degree. to 1500.degree. C. Between 0.1 and 10 mol % of at least one of the ammonium halogenides is added per mol of the Al present to the product thus obtained, and the mixture thus obtained is subsequently subjected to a second heat treatment at a temperature of 700.degree. to 1200.degree. C.