Four commercial brands of infant milk formula were investigated, and the amount of shelf lipid peroxidation was determined by measuring malondialdehyde (MDA) or, more exactly, thiobarbituric acid reactive substance (TBARS) levels, which are the end products of lipid peroxidation. TBARS level, indicated by MDA concentration, was higher in the vegetarian brands. The vegetarian brands are, as expected, more prone to oxidation than dairy brands, as they contain more unsaturated fats. The introduction of formate anion diminishes the peroxide amounts initially present in the milk samples as well as those induced by radiation-induced oxidation.