Hemocyanins (Hcs) are large glycoproteins present in the blood of some mollusks and arthropods, whose biological function is mainly related with the oxygen transport to the tissues. In addition, mollusk Hcs are used as carriers/adjuvants and non-specific immunostimulants in biomedicine. Achieving structural stabilization in proteins having therapeutic application is an important task. In the present study, the carbohydrate moieties of the hemocyanin, isolated from the garden snail Helix aspersa maxima and its isoforms, were oxidized with sodium periodate. The changes in the microenvironment around the chromophores in the protein molecules and the structural stability were evaluated and discussed. The oxidized proteins demonstrated increased resistance to proteolytic cleavage, as well as enhanced thermal stability.