The tripler state cleavage of m-iodo- (mIBP) and p-iodobenzophenone (pIBP) to benzoylphenyl and iodine radicals has been studied by both steady state and laser flash techniques. Phosphorescence spectra in methanol/ethanol glasses at 77 K indicate triplet energies of 68.4 and 67.8 kcal/mol for mIBP and pIBP, respectively. In most organic solvents products are benzophenone and iodinated solvent; these are formed by hydrogen abstraction from solvent by the initially formed benzoylphenyl radicals. Quantum yields for product formation decrease with increasing solvent viscosity, ranging from 0.45 in acetonitrile to 0.28 in cyclopentane and 0.05 in cyclooctane; they appear to reflect significant in-cage recoupling of the initial radical pairs. Quenching of the reaction by naphthalene indicates 25 degrees C tripler lifetimes of 25 and 0.2 ns for mIBP and pIBP, respectively, which were confirmed by flash kinetics. Laser excitation produced transient signals assigned to the iodine atom-toluene complex (in toluene) and, for mIBP, the triplet ketone. Decay of the latter revealed activation parameters Delta H double dagger = 4.0 +/- 0.2 kcal/mol and log A 10.6 +/- 0.2 for mIBP in both methanol and toluene. The strong phosphorescence of both ketones at 77 K indicates that Delta H double dagger for pIBP must be greater than or equal to 3 kcal/mol, such that log A is similar to 12, much higher than for mIBP. The different rates and A values reflect different pi* spin densities at the meta and para positions in the lowest n,pi* triplets, which react either by mixing with dissociative sigma,pi* states or by thermally converting into a dissociative sigma,sigma* or pi(I),sigma* state. The dependence on pi* spin density and the low A values strongly suggest an inefficient state interconversion at a surface crossing between n,pi* and sigma,sigma* states, because of their different symmetries. It is suggested that the low A values, which might indicate negative Delta S double dagger values, are better understood as low transmission coefficients.