The actinyl ions, consisting of the dioxoactinde(VI) and dioxoactinide(V) ions, are unique in the periodic table. However, very few quantitative studies of the spectra of these ions have been performed. In this paper the analysis of the optical spectrum of the aqueous neptunyl(V) [NpO2+] ion in 1 M HClO4 is reexamined. The species in solution is assumed to be the linear NpO2+ ion surrounded in the equatorial plane by five oxygen atoms from five H2O molecules. The neptunyl(V) ion has the 5f(2) open-shell configuration, and the low-lying optical transitions (up to similar to 20 000 cm(-1)) observed are primarily from the two 5f(1) electrons occupying the 5f phi and 5f delta orbitals. The conventional parametric theory used for fn systems is applied to these low-lying transitions utilizing the intensity calculations that Matiska et al.(1) have performed for this ion and the data reported by Eisenstein and Pryce.(2) Possible ratios for the Slater electrostatic repulsion parameters are obtained from the data for the isoelectronic ion U4+ (5f(2)) in various host crystals. The results are consistent with earlier crystal field analyses of the 5f(1) neptunyl(VI) [NpO22+] ion.