Abstract: Hemoglobin for forming the basis of blood substitute is modified by crosslinking it by reaction with a polyaldehyde produced by ring opening oxidation of a di/tri-saccharide such as raffinose, wherein the o-saccharide solutions are maintained and the reaction is conducted at a pH from about 5.0-7.0, and at a stoichiometry from about 1:1-4:1 based upon the molar ratio of di/tri-saccharide to hemoglobin tetramers, followed by appropriate reduction of the Schiff base linkages so formed to stabilize them. Careful control of the pH of ring opening of the di/tri-saccharide ensures consistent production of the polyaldehyde without unwanted side products and reaction of this product with the hemoglobin at the specified stoichiometry leads to high yields of stabilized, tetrameric hemoglobin and oligomers thereof of predetermined composition and beneficial properties.