Abstract The formation of ferryl heme (Fe(IV) = O) species, i.e., compound I and compound II, has been identified as the main intermediates in heme protein peroxidative reactions. We report stopped-flow kinetic measurements which illustrate that the reaction of hemoglobin I (HbI) from Lucina pectinata with hydrogen peroxide produce ferryl intermediates compound I and compound II. Compound I appears relatively stable displaying an absorption at 648 nm. The rate constant value (k′2) for the conversion of compound I to compound II is 3.0 × 10−2 s−1, more than 100 times smaller than that reported for myoglobin. The rate constant value for the oxidation of the ferric heme (k′12 + k′13) is 2.0 × 102 M−1 s−1. These values suggest an alternate route for the formation of compound II (by k′13) avoiding the step from compound I to compound II (k′2). In HbI from L. pectinata the stabilization of compound I is attribute to the unusual collection of amino acids residues (Q64, F29, F43, F68) in the heme pocket active site of the protein.