The tetranuclear complex [Mn-II(2-OH-picpn)](4+)(4) catalytically disproportionates H2O2 to give H2O and O-2. Saturation kinetics are observed as well as a significant deuterium isotope effect when D2O2 is used as substrate. While this compound exhibits one of the largest V-max values for a manganese catalase mimic, the k(cat)/K-M values are similar to those of other highly reactive manganese models. A speculative model based on iron chemistry may explain the low binding affinity of peroxide for the compound.