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Previously prepared Mn(II)- and quinol-containing magnetic resonance imaging (MRI) contrast agent sensors for H 2 O 2 relied on linear polydentate ligands to keep the redox-activatable quinols in close proximity to the manganese. Although these provide positive T 1 -weighted relaxivity responses to H 2 O 2 that result from oxidation of the quinol groups to p -quinones, these reactions weaken the binding affinity of the ligands, promoting dissociation of Mn(II) from the contrast agent in aqueous solution. Here, we report a new ligand, 1,8-bis(2,5-dihydroxybenzyl)-1,4,8,11-tetraazacyclotetradecane, that consists of two quinols covalently tethered to a cyclam macrocycle. The macrocycle provides stronger thermodynamic and kinetic barriers for metal-ion dissociation in both the reduced and oxidized forms of the ligand. The Mn(II) complex reacts with H 2 O 2 to produce a more highly aquated Mn(II) species that exhibits a 130% greater r 1 , quadrupling the percentile response of our next best sensor. With a large excess of H 2 O 2 , there is a noticeable induction period before quinol oxidation and r 1 enhancement occurs. Further investigation reveals that, under such conditions, catalase activity initially outcompetes ligand oxidation, with the latter occurring only after most of the H 2 O 2 has been depleted.