Since hyperglycaemia changes fluidity of erythrocyte cell membrane and impair cell deformability, our goal was to characterize light refractive properties of haemoglobin and red blood cells (RBC) in diabetes patients. Microscopic investigation was carried out on intact and in methanol fixed RBCs of diabetes patients with long-term hyperglycaemia (glycosylated haemoglobin > 7,5%). Interference microscopy was used for refractive index (RI) measurements at 18 different pH levels in range of 2 - 13. Results showed that the curves of RI in intact and fixed cells of diabetes patients and fixed cells for control group were of similar configuration, with one branch in alkaline portion, one branch in acidic portion, and two minima and a maximum in neutral portion of pH scale. The curves of RI of intact cells of the control group were with one minimum in the neutral portion of pH scale. The curves of the individuals from the control group overlapped each other (maximum for fixed cells is at pH equals 6,3; minimum for intact cells at pH equals 7,2). On the contrary, curves of RI in RBC of diabetes patients were not uniform in the neutral portion and alkaline portion (maximum for fixed and intact cells at pH equals 6,9). The curves of RI in RBC of diabetes patients in the neutral zone was shifted towards the alkaline end of the pH scale, and the curves of RI of RBC were lower compared to the control curves.