Competitive kinetics were applied as a tool to determine apparent rate constants for the reduction of hypervalent haem pigment ferrylmyoglobin (MbFe(IV)=O) by proteins and phenols in aqueous solution of pH 7.4 and I = 1.0 at 25 °C. Reduction of MbFe(IV)=O by a myofibrillar protein isolate (MPI) from pork resulted in k_MPI = 2.2 ± 0.1 × 10⁴ M^-1 s^-1. Blocking of the protein thiol groups on the MPI by N-ethylmaleimide (NEM) markedly reduced this rate constant to k_MPI-NEM = 1.3 ± 0.4 × 10³ M^-1 s^-1 consistent with a key role for the Cys residues on MPI as targets for haem protein-mediated oxidation. This approach allows determination of apparent rate constants for the oxidation of proteins by haem proteins of relevance to food oxidation and should be applicable to other systems. A similar approach has provided approximate apparent rate constants for the reduction of MbFe(IV)=O by catechin and green tea extracts, though possible confounding reactions need to be considered. These kinetic data suggest that small molar excesses of some plant extracts relative to the MPI thiol concentration should afford significant protection against MbFe(IV)=O-mediated oxidation.