Several bromophenyl alkyl ketones undergo photoinduced radical cleavage of bromine atoms; the resulting acylphenyl radicals are trapped as the dehalogenated ketones either by alkane solvents or by added thiols. Quantum yields for this process decrease with increasing solvent viscosity; this behavior indicates that recoupling of the phenyl/bromine bromine radical pairs competes with their diffusion apart. In some cases Norrish type II elimination competes with the cleavage reaction. Steady state studies indicate room temperature rate constants for triplet state C-Br radical cleavage of 2 and 1 x 10(8) s(-1), respectively, for m- and p-bromoacetophenone. Flash kinetics measurements of the temperature dependence for their triplet decay provided activation parameters for triplet state cleavage : Delta S-double dagger = -5 eu for the para isomer and -3 eu for the meta; Delta H-double dagger = 5.3 kcal/mol for both. The values for the meta isomer are the same in methanol and toluene and extrapolate to the room temperature decay rate measured by steady stale techniques. The 0.002-0.003 quantum yields for conversion of m- and p-bromobenzophenone to benzophenone in cyclopentane solvent indicate triplet state cleavage rates of 0.8-1.0 x 10(5) s(-1), which would suggest Delta H-double dagger values near 10 kcal/mol. Consideration of triplet excitation energies and of the phenyl-Br bond strength suggests a mechanism for triplet cleavage involving activated C-Br bond stretching in the triplets until a pi,pi* configuration is converted into a dissociative n(Br)(pi),sigma* state. It is suggested that the different isomers have comparable mixing of n(Br)(pi),pi* character into their pi,pi* triplets and that the state interconversions occur inefficiently at surface crossings with a n(Br)(pi),sigma* state. The o-bromo ketones show unusual behavior. The Arrhenius plot for triplet decay of o-bromoacetophenone is curved and only half the total cleavage can be quenched by naphthalene. o-Bromobenzophenone is 500 times more reactive than its isomers; direct interaction between bromine and carbonyl as well as steric depression of the C-Br dissociation energy may both contribute to this unusual reactivity.