An alkali metal aluminosilicate of the zeolite P type is manufactured by continuous mixing of a sodium silicate solution and a sodium aluminate solution in a first reaction zone in the presence of zeolite P seeds, continuously transferring this mixture to a second reaction zone, and filtering, washing, and drying the resulting product. The aluminosilicates are of value as components in detergent formulations for removing calcium and magnesium hardness ions by ion exchange.