Non-enzymatic glycation, accompanied by the formation of free radicals, represents a serious problem in diabetes mellitus. It is supposed to be the cause of the development of long-term diabetic complications. The aim of this work was to estimate the effect of treatment with vitamin C (1 g per day) and E (600 mg per day) on selected biochemical parameters as well as to determine the physicochemical state of erythrocyte membranes in diabetics. The paper also compares the physicochemical state of diabetic and control erythrocyte membranes. The changes in the values of glycaemia, glycated haemoglobin, and fructosamine were insignificant after three months of treatment. This points out that the doses used could be low or that the patient compliance was poor. An anionic fluorescent probe merocyanine 540 (MC540) was used to monitor possible changes in the physicochemical properties of isolated diabetic erythrocyte membranes. Significantly higher affinity of MC540 monomers to the membrane in diabetics treated with vitamin E was observed, which can be the result of the antioxidative effect of the vitamin (p < 0.02). A comparison of absorption spectra of MC540 in diabetic and control membranes revealed significant changes in the position of the bands and in their absorbances (p < 0.01 and less). They result from substantial alterations in the structure, surface charge, and the fluidity of erythrocyte membranes in diabetes mellitus. (Tab. 2, Fig. 3, Ref. 22.)