Polycrystalline samples of Nd18Li8Fe5-xMxO39 (M = Mn, 0 < x <= 4; M = Co; 0 < x <= 3) have been synthesized using a solid-state method and have been studied using a combination of dc and ac magnetometry, neutron diffraction, and Mossbauer spectroscopy. All compositions adopt a cubic structure (space group Pm (3) over barn, a(0) similar to 11.9 A) based on intersecting < 111 > chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Nd-O framework. The trigonal prismatic site is occupied by Li+ and, in some cases, high-spin Fe3+. When M = Mn, the smaller of the two distinct octahedral sites in the structure is occupied by a disordered distribution of Mn4+ and low-spin Fe4+, but in Nd18Li8Fe4CoO39, this site is occupied exclusively by Fe4+. The larger of the octahedral sites is occupied by a disordered distribution of M3+, high-spin Fe3+, and, in some cases, Li+. Unusually, the Mn3+ cations in Nd18Li8Fe5-xMxO39 adopt a low-spin state. This is attributed to the presence of an internal chemical pressure generated by the Nd-O framework. Competing magnetic superexchange interactions lead to the formation of a spin-glass-like phase in all compositions below T-f similar to 5 K; the transition temperature decreases with increasing cobalt content but is relatively insensitive to the composition when M = Mn.