Myoglobin (Mb) from gastropod mollusc Aplysia limacina shows only 20% sequence homology to the ‘prototype’ sperm whale Mb but exhibits a typical Mb fold and can reversibly bind oxygen. An intriguing feature of aplysia Mb is that it lacks the distal histidine and displays a ligand stabilisation based on an arginine. Here we report the reaction of aplysia metMb with hydrogen peroxide studied by optical and electron paramagnetic resonance (EPR) spectroscopies. Two electron oxidation of the protein by H₂O₂ results in formation of two intermediates typical for this class of reactions, the oxoferryl haem state and a globin-bound free radical. An unusual characteristic of the aplysia Mb reaction is formation, prior to haem oxidation, of an optically distinct compound with an EPR spectrum typical of the low spin Fe³⁺ haem state. This compound is interpreted as the complex between H₂O₂ and the ferric haem state (Compound 0), formed prior to cleavage of the dioxygen bond. We conclude that H₂O₂ is singly deprotonated in Compound 0 which can thus be notated as [Fe³⁺–⁻OOH]. A new low spin ferric haem state has been observed over the period of Compound 0 decay, and hypotheses have been formulated as to its identity and role. The location of the protein bound radical observed in aplysia Mb is discussed in light of the fact that the protein does not have any tyrosine residues, the most common site of free radical formation in the haem protein/peroxide systems. All intermediates of the reaction are kinetically characterised.