We measured the circular dichroism (CD) and absorption spectra of the B-band region of microperoxidase 11 (MP11) as a function of temperature and peptide concentration. At micromolar concentrations, small MP11 dimers or trimers lead to excitonic coupling between low-spin and high-spin heme groups, to which the NH(2) group of the MP11 N-terminal and H(2)O are bound as a sixth ligand, respectively. These aggregates convert into monomers with hexacoordinated high-spin heme groups with increasing temperature. This transition can be described by a two-state model. Aggregation becomes more extended at 50 microM concentration and causes some B-band hyperchromism, which reflects a J-type arrangement of heme groups linked together in the aggregates formed. At near-millimolar concentration, the CD and absorption spectra of the B-band region suggest the existence of even more extended and thermally stable aggregates, which might involve mu-oxo dimers of the heme groups. The degree of aggregation at 50 and 500 microM concentration increases substantially if the sample is freed from most of its oxygen in a N(2) atmosphere. The CD spectrum of the monomeric high-spin species is reminiscent of that observed for the unfolded alkaline conformation of the intact protein. Finally, we investigated the binding of acetylmethionine (AcM) ligands to the heme at aggregation-supporting conditions (500 microM concentration). The data suggest that the ligand prevents any substantial aggregation. As a surprising result, our data reveal that AcM-MP11 complexes exhibit a high-spin/low-spin mixture, with the high-spin configuration being stabilized at high temperatures.