The Na-2(+) ion was produced in neon plasma and isolated in solid neon st 4 K. Its X-band electron-spin-resonance (ESR) spectrum was analyzed to yield the isotropic hyperfine interaction of 441 MHz with the Na-23 (l = 3/2) nucleus. An approximate interpretation of this value points to considerable p sigma character in the predominantly s sigma wave function of the single valence electron in the ground (2)Sigma(g)(+) state.