We have measured kinetic-energy-release distributions (KERD) for spontaneous dissociation of electronically excited dimer ions of krypton and xenon, formed by electron impact ionization of neutral precursors. The data cannot be reconciled by decay of the strongly bound II(1/2u) state that successfully explains dissociation of Ne-2(+) and Ar-2(+). Instead, the KERD is dominated by contributions from the weakly bound II(1/2g) state that has so far escaped a convincing experimental characterization. The present data can be utilized to assess the accuracy of ab initio potential energy curves of this state. (c) 2007 Elsevier B.V. All rights reserved.