The peroxidase activity of haemoglobin A was characterized for non-glycated and glycated haemoglobin (HbA1) within the pH range 4.5 to 6.0, by measuring the rate of oxidation of 5-aminosalicylic acid following the degradation of H2O2. Glycation was found to significantly lower the pH activity of haemoglobin peroxidase throughout the pH range. However, in the presence of 100 mmol/l sorbitol the pH activity profile of glycated haemoglobin was significantly elevated whilst that of non-glycated haemoglobin remained unchanged.