Complexes of 1,10-phenanthroline (phen) with Np(V) and Pu(V) benzoates of the compositions NpO2(phen)(OOCC6H5) and PuO2(phen)(OOCC6H5) were synthesized. A powder X-ray diffraction study showed that these compounds, depending on the preparation conditions, exist in the form of two phases having essentially the same composition but different powder patterns. The phases isolated from hot (70–100°C) solutions are isostructural with the previously described complexes AnO2(bipy)(OOCC6H5) (An = Np and Pu, bipy = 2,2′-bipyridine), i.e., in their structure there are dimeric (AnO2)22+ cations formed by mutual coordination of two AnO2+ ions via “yl” oxygen atoms. The compounds AnO2(phen)(OOCC6H5) prepared by slow crystallization in the cold or at weak (up to 45°C) heating are isostructural with each other but appreciably differ in the structure from the high-temperature phases. The electronic absorption spectra of the compounds and their thermal behavior were examined.