Crystals of the title compound, octaammonium dicerium octasulfate tetrahydrate, suitable for X-ray work, were prepared by a hydrothermal reaction. The structure is made up of [Ce2(SO4)8]8- anions, interacting with the crystallization waters and the NH4+ cations through hydrogen bonds. The CeIV atom in the [Ce2(SO4)8]8- anion is ninefold coordinated by O atoms belonging to sulfate groups. The coordination polyhedron is a somewhat distorted tricapped trigonal prism. The Ce-O distances range from 2.325(2) to 2.478(2) A.