Efficiently separating the chemically similar lanthanide ions into elementally pure compositions is one of the greatest scientific challenges of the 21st century. Although extensive research efforts have focused on the development of organic extractants for this purpose, the implementation of aqueous complexants possessing distinct coordination chemistries has scarcely been explored as an approach to enhancing intralanthanide separations. In this study, we investigate the lanthanide coordination chemistry of macrophosphi, a novel analogue of the reverse-size selective expanded macrocycle macropa. Our studies reveal that substitution of the pyridyl-2-carboxylic acid pendent arms of macropa with pyridyl-2-phosphinic acid arms of macrophosphi gives rise to a dramatic enhancement in the ability to discriminate between light lanthanides, reflected by a binding affinity of macrophosphi for La3+ that is over 5 orders of magnitude higher than that for Gd3+. Furthermore, upon implementation of macrophosphi as an aqueous complexant in a biphasic extraction system containing the industrial extractant bis(2-ethylhexyl)phosphoric acid, separation factors of up to 45 were achieved for the Ce/La pair. These results represent a remarkable separation of adjacent lanthanides, demonstrating the significant potential of reverse-size selective aqueous complexants in lanthanide separation schemes.