The protein known as dehaloperoxidase-hemoglobin (DHP) from the terebellid polychaete Amphitrite ornata has provided us with an example of a multi-functional protein that challenges many of the assumptions behind the structure–function correlation owing to its dual roles as an oxygen transport globin and a peroxidase. In this chapter, we will present an overview of the peroxidase function of DHP, and will describe the mechanism of the oxidative dehalogenation reaction carried out by this enzyme when initiated from both the ferric and oxyferrous states by the addition of hydrogen peroxide. The structural and electronic properties of the heme active site will also be discussed in the context of the proximal and distal cavities and the corresponding charge relays. The question of small molecule binding will be addressed, with a particular emphasis on the inhibitor binding site and the current understanding of internal versus external substrate binding. Finally, the deactivation of DHP and its physiological role will be presented.