We have studied the photodissociation dynamics of expansion-cooled IO radicals at 454.9 nm, corresponding to the A(2) Pi(3/ 2) - X-2 Pi(3/2) (1-0) bandhead, using velocity map ion imaging. The radicals were generated using a late-mixing dual-valve photolytic reactor source. Analysis of the I(P-2(3/ 2)) photofragment speed distribution provides a direct determination of the ground state bond energy of D-0(0)(IO) = 54.9(-0.4) (+0.2) kcal/ mol and a heat of formation 298K of Delta H-f (298)(IO) = 29.2(-0.2)(+0.4) kcal/ mol. (c) 2008 Elsevier B. V. All rights reserved.