The paper describes a general method for determining absolute branching ratios in mass spectrometric experiments. The method overcomes the chronic obstacle that the daughter ion fragmentation pattern of radical products is usually unknown. We report the absolute product branching ratio for competing primary C-Cl and C-C bond fission in chloroacetone after excitation on the (1)n pi* absorption band. To determine this branching ratio, acetyl chloride is used to calibrate the relative detection efficiency of acetyl radicals at the CH2CO+ daughter ion relative to Cl atoms at Cl-35(+). We also calculate the daughter ion production probability for CH2CO+ formed from internally excited CH3CO radicals.