Process for manufacturing an alkali metal aluminosilicate of zeolite wherein zeolite P seeds are produced in situ in a first reacting gel with a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of above 2.2 and it is only after the formation of zeolite P seeds, but before said seeds have the time to fully grow, that sodium aluminate is added in order to get a gel which can have a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 2 which therefore enables producing a zeolite P having also a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 2.