This study investigated how silver nanoparticles (AgNP) and ionic silver (AgNO3) undergo phase transformations in soils under aerobic and anaerobic conditions using extended X-ray absorption fine structure (EXAFS) spectroscopy. After 30 days of aerobic incubation, 88% of AgNP added to the soil remained persistent, whereas AgNO3 was completely transformed into Ag associated with humus and clay minerals. In the anaerobic soil, 83% of the spiked AgNP was transformed into Ag2S, accompanied by significant decrease in water- and acid-extractable Ag fractions. About 50% of AgNO3 spiked to the anaerobic soil underwent transformations into metallic Ag and associations with clay minerals. Oxide (Ag2O) and carbonate (Ag2CO3) forms of Ag were not predominant in aerobic and anaerobic soils. The redox potential of soil had a profound effect on determination of the phase-transformation pathways for AgNP and ionic Ag. (C) 2015 Elsevier B.V. All rights reserved.