The effect of pH on iron-containing complex coacervate core micelles [Fe(III)-C3Ms] is investigated in this paper. The Fe(III)-C3Ms are formed by mixing cationic poly(N-methyl-2-vinylpyridinium iodide)-b-poly(ethylene oxide) [P2MVP(41)-b-PEO(205)] and anionic iron coordination polymers [Fe(III)-L(2)EO(4)] at stoichiometric charge ratio. Light scattering and Cryo-TEM have been performed to study the variations of hydrodynamic radius and core structure with changing pH. The hydrodynamic radius of Fe (III)-C3Ms is determined mainly by the corona and does not change very much in a broad pH range. However, Cryo-TEM pictures and magnetic relaxation measurements indicate that the structure of the micellar cores changes upon changing the pH, with a more crystalline, elongated shape and lower relaxivity at high pH. We attribute this to the formation of mixed iron complexes in the core, involving both the bis-ligand and hydroxide ions. These complexes are stabilized toward precipitation by the diblock copolymer.