Nonadiabatic Monte Carlo transition-state theory is used to explore competing C-Cl and C-Br bond fission processes in a simple model of (1)[n,pi *CO)]photoexcited bromoacetyl chloride. Morse potentials are used to represent bond stretching coordinates, and the positions and magnitudes of nonadiabatic coupling between excited state potentials are modeled using ab initio data. The main effect of nonadiabaticity is to favor C-Cl fission over C-Br, despite a larger barrier to C-CI dissociation. The absolute values of the rate constants are smaller than observed experimentally, but the calculated branching ratios are close to the experimental value. For C-Cl fission, it is shown that the minimum energy crossing point is not sufficient to describe the rate constant, suggesting that care must be taken when using alternative models which make this assumption. (C) 2001 American Institute of Physics.