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The chlorite product of horseradish peroxidase, compound X, is shown by magnetic circular dichroism (MCD) spectroscopy in the temperature range 1.6–50 K to have a very similar haem structure to compound II under the same conditions (pH 10.7). Both are concluded to contain the Fe(IV)=0 group. The MCD spectrum also detects an unusual species, absorbing at wavelengths between 600 and 750 nm, that has magnetic properties different from those of the ferryl haem group. It is suggested that this is a species at the same oxidation level as ferryl haem but with the porphyrin ring having suffered a one-electron oxidation, i.e. [Fe(III) P .+].