The title complex, [Ce(C2H6OS)8]2[Mo6O19]3·4C2H6OS, was obtained as a byproduct of the reaction of [(C4H9)4N]2[Mo6O19] with Ce(NO3)3·6H2O and phthalic acid in dimethylsulfoxide solution. The asymmetric unit consists of a complex [Ce(C2H6OS)8]3+ cation, one and a half of the Lindqvist-type [Mo6O19]2− polyanions and two dimethylsulfoxide solvent molecules; the half polyanion lies on an inversion center. The Ce3+ ion is coordinated by eight dimethylsulfoxide ligands through the O atoms in the form of a distorted square antiprism. The Ce—O bond lengths range from 2.429 (6) to 2.550 (5) Å. The cohesion of the structure is ensured by S⋯O [3.115 (6), 3.242 (10) and 3.12 (3) Å], O⋯O [3.037 (10) Å] and C—H⋯O interactions between cations and anions. The S and C atoms of a dmso ligand are disordered over three sites in a 0.45:0.30:0.25 ratio. The dimethylsulfoxide solvent molecules are highly disordered and could not be modelled successfully; their contribution was therefore removed from the refinement using the SQUEEZE routine in PLATON [Spek (2009 ▶). Acta Cryst. D65, 148–155]. Potential solvent-accessible voids of 500.0 Å3 occur in the crystal structure.