Li11Nd18Fe4O39-delta has been synthesized by the solid-state reaction of pellets, covered with powder of the same composition to avoid lithium loss, with a final reaction temperature of 950 degrees C. This phase has been reported previously to have various stoichiometries: Li5Nd4FeO10, Li8Nd18Fe5O39, and Li1.746Nd4.494FeO9.493. The crystal structure of Li11Nd18Fe4O39-delta is closely related to that reported previously for two of the other three compositions but contains extra Li and differences in Li/Fe site occupancies. Fe is present in a mixture of 3+ and 4+ oxidation states, as confirmed by Mossbauer spectroscopy. The oxygen content of 39 - delta is variable, depending on the processing conditions. Samples slow-cooled in air from 800 degrees C are semiconducting, attributed to the presence of Fe4+ ions, whereas samples quenched from 950 degrees C in N-2 are insulating.