A new 0(u)(+)6s Rydberg state built on an excited ionic core of I-2 has been observed by optical triple resonance via the B(O-u(+)) valence and EOg+ (P-3(2)) ion-pair states with ionization detection. A vibrational progression, v = 0-27, was observed for selected rotational levels and molecular constants are presented. The Rydberg state is weakly bound and was determined to have an internuclear separation of 4.500 +/-0.003 angstrom. (c) 2005 Elsevier B.V. All rights reserved.