The stability (half-life, t(A1/2)) of the large catalase (CAT) isolated from Aspergillus terreus was decreased under acidic conditions (maximum t(A1/2) similar to 8.5 months at pH a parts per thousand currency sign 6) versus alkaline conditions (t(A1/2) similar to 15 months at pH 8-12). Acidic conditions induce the dissociation of haem from CAT, as revealed from a reduction in the Soret peak intensity at 405 nm and an increase in the peak current at Fe3+/Fe2+ redox potentials. This increase in current is attributed to the facile electron transfer from the free haem generated on the electrode surface as a result of its disintegration from the insulating protein matrix. The haem isolated from CAT at acidic condition was reconstituted with apo-CAT at alkaline denaturing conditions to regenerate the CAT activity.