Polycrystalline samples of Nd18Li8Fe5O39 and Nd18Li8Co4O39 have been synthesized using a solid-state method and studied by a combination of neutron diffraction, direct and alternating current magnetometry, and, in the case of Nd18Li8Fe5O39, Mossbauer spectroscopy. Both compounds adopt a cubic structure (space group Pm (3) over barn, a(0) similar to 11.9 angstrom) based on intersecting < 111 > chains made up of alternating octahedral and trigonal-prismatic coordination sites. The Fe4+ cations in Nd18Li8Fe5O39 are found on only the smaller of the two distinct octahedral sites in the structure; Fe3+ and Li+ are disordered over the larger octahedral site and the trigonal-prismatic site. The Nd3+ cations occupy sites between the chains. The smaller octahedral site is fully occupied by cobalt in Nd18Li8Co4O39, with 25% of the larger octahedral sites being vacant; Li+ is only found on the prismatic sites. Nd18Li8Fe5O39 shows spin-glass-like behavior with a spin-freezing temperature of 5.75 K, whereas Nd18Li8Co4O39 appears to order antiferromagnetically at 2.3 K. In both cases, the magnetic coupling involves the Nd3+ sublattice.