Uranyl peroxide polyhedra are known to self-assemble into complex closed clusters with fullerene and other topologies containing as many as 60 polyhedra. Here clusters containing 20 uranyl pentagonal triperoxides have been isolated and characterized that assume the smallest possible fullerene topology consisting only of 12 pentagons. Oxalate has been used to crystallize fragments of larger uranyl peroxide clusters, and these fragments and other known structures indicate that the U-O-2-U dihedral angle is inherently bent. Such bending is thought to be essential in directing the self-assembly of uranyl peroxide polyhedra into closed clusters.