1. The oxidation of deuteroferrihaem by H2O2 to bile pigment and CO was studied both by stopped-flow kinetic spectrophotometry and mass spectrometry, at 25°C, I=0.1m. 2. Spectrophotometric studies imply that, at constant pH, the rate of bile pigment formation is first-order with respect to [H2O2] and also proportional to [deuteroferrihaem monomer]. The effect of pH on the apparent second-order rate constant suggests that acid-ionization of deuteroferrihaem monomer is important in the reaction mechanism. 3. The relative rates of formation of O2 (from catalytic decomposition of H2O2) and CO (from oxidation of ferrihaem) have been measured by mass spectrometry. The results are in excellent agreement with those obtained by combining kinetic data for catalytic decomposition (Jones et al., 1973, preceding paper) with the spectrophotometric results for deuteroferrihaem oxidation.