Laboratory proton nuclear magnetic resonance (NMR) relaxation measurements were made on fluid-saturated soil samples under reducing and oxidizing conditions. We used well-characterized kaolinite and sand samples with known concentrations of Fe(II) and Fe(III) compounds as the representative soils. Changes in dissolved oxygen concentration in the water in the soils caused changes in relaxation time that are too small to be reliably detected by field NMR measurements. In contrast, NMR relaxation measurements were shown to be very sensitive to small changes in the concentration of Fe(III) species in soil, as a consequence of changing redox conditions. Oxidation of as little as 0.030 mg/g of Fe(II) species to Fe(III) oxyhydroxides resulted in a 30--50% decrease in relaxation time. These results have important implications for the potential use of NMR field instruments to monitor changing redox conditions using the Fe(III)-Fe(II) redox couple as an indicator.