Fractional iron absorption from a breakfast meal was determined in Peruvian children employing stable iron isotopes as labels. Iron isotopic analysis was performed by the recently developed negative thermal ionization technique for high-precision iron isotope ratio measurements using FeF4- ions. By increasing the ascorbic acid content of the standard breakfast meal as served within the Peruvian school-breakfast program from 27 mg to 70 mg, it was possible to increase the geometric mean fractional iron absorption significantly from 5.1% (range 1.6-13.5%) to 8.2% (range 3.1-25.8%). Fractional iron absorption was calculated according to isotope dilution principles and by considering the non-monoisotopic character of the used spikes. © Springer-Verlag 1997.