Understanding the phase distribution of nanoparticles between soils or sediments and water is a critical factor in determining their fate in environmental systems. As such, we examined the interactions of ...-labeled multiwalled carbon nanotubes (MWNTs) in aqueous systems with peat as a model solid organic material. MWNTs that had been treated with strong oxidative acids possessed negative charges on their surfaces and were relatively stable in deionized water. In the absence of peat, MWNTs aggregated when the concentration of sodium cations was above 4.0 mM or at a solution pH of 4.0. Dissolved organic matter (DOM) from peat effectively stabilized MWNTs in solution by making the suspended nanotubes less sensitive to changes in sodium concentration or solution pH. Direct sorption interactions between MWNTs and solid peat were not observed in the absence of sodium cations, whereas they became apparent when the ionic strength was sufficiently high to reduce electrostatic repulsion between peat and MWNTs. (ProQuest: ... denotes formulae/symbols omitted.)