Iron deficiency in peanuts (Arachis hypogeae L.) caused an increase in release of caffeic acid, a higher rate of Fe(III) reduction, and increased rates of both Fe(III) chelate splitting and iron uptake.Experiments on Fe(III) reduction by phenolics (in vitro experiments) and by roots of Fe-deficient peanuts exclude the direct involvement of released phenolics in Fe(III) reduction by roots: Fe(III) reduction by phenolics had a pH optimum higher than 8.0 and was strongly dependent on the concentration and the stability of the supplied Fe(III) chelates. In contrast, Fe(III) reduction by roots of Fe-deficient peanuts had a pH optimum of about 5.0 and was less dependent on the stability of the supplied Fe(III) chelates. Furthermore, the observed release of phenolics into nutrient solution would have to be at least 200 times higher to attain the reduction rates of roots of Fe-deficient peanuts. The results of these experiments support the idea of an enzymic reduction of Fe(III) on the plasmalemma of cortical cells of roots.