Oxidation is not only one of the major degradation pathways during storage of proteins and peptides but also a potential problem in protein production, isolation and purification processes. Spontaneous oxidation is accelerated by light and transition metal ions, while contaminating oxidants derived from various unexpected sources are also possible factors causing oxidation. A number of excipients have been tested for their protective effect against oxidation. Porcine lactate dehydrogenase (LDH) was used as a model protein, oxidation of which was performed using copper ions and hydrogen peroxide, separately and also in combination. Oxidation and denaturation of the enzyme was followed as activity loss, modification of certain amino acids, altered secondary structure and aggregation. The polycation, polyethyleneimine (PEI) and EDTA contributed to maintenance of enzyme activity and conformation during both modes of oxidation. The melting temperature profiles of the enzyme indicated that addition of both PEI and sorbitol resulted in increased stability of LDH towards denaturation by both oxidation and temperature.