Pulsed laser photolysis combined with laser induced fluorescence of IO has been used to study the photofragment excitation spectrum of OIO. Analysis of the O- IO dissociation threshold observed around 480 nm provides the first experimental determinations of the OIO bond dissociation energy and heat of formation at 298 K, which are given by D-0 degrees(O-IO) = (248.8 +/- 1.0) kJ mol(-1) and Delta H-f(298)degrees(OIO) = (118.5 +/- 2.0) kJ mol(-1), respectively. (C) 2009 Elsevier B. V. All rights reserved.