Infant cereals and formulas are usually fortified with iron to prevent iron deficiency. To enhance iron bioavailability, supplemental ascorbic acid is recommended. Ascorbic acid is considered to be an antioxidant in vivo, but has pro-oxidant effects when exposed to non-protein-bound iron. We measured formation of free radicals in cereals and infant formulas after addition of ascorbic acid. The production of hydroxyl radicals was assessed by hydroxylation of salicylic acid to 2,5-dihydroxybenzoic acid (2,5-DHBA). Production of 2,5-DHBA increased with increasing ascorbic acid doses added. Addition of 0.8 mM ascorbic acid to breast milk produced less radicals (0.03 ± 0.05 μM) than addition of ascorbic acid to low-iron formula (0.13 ± 0.08 μM, P = 0.019), medium-iron formula (0.34 ± 0.12 μM, P < 0.0001) or high-iron formula (0.44 ± 0.08 μM, P < 0.0001). Even when iron content in breast milk was adjusted to a level comparable with that of formulas, production of 2,5-DHBA was lower. Breast milk seems to contain substances that reduce hydroxyl radical formation.