Abstract-Nonylphenol ethoxylate (NPEO x ) homologs present in commercial mixtures were rapidly adsorbed to and desorbed from native river sediment. Adsorption isotherms, established using high-performance liquid chromatography analysis to monitor individual homologs NPEO 3 to NPEO 13 simultaneously, were linear for each component adsorbing to native sediment, organic-free sediment, and kaolinite, usually with small positive intercepts on the isotherms indicating an additional low-capacity, high-affinity binding site. The adsorption partition coefficients (K d ) for the native sediment decreased progressively from 1,460 L/kg for NPEO 3 to 450 L/kg for NPEO 10 , then increased again slightly for higher homologs. In contrast, K d values for organic-free sediment (range 230-590 L/kg) or kaolinite (190-490 L/kg) increased steadily from NPEO 3 to NPEO 13 . Adsorptions to silica and alumina were very weak, but to sewage sludge all components adsorbed strongly (K d values 12,000-33,000, with a maximum at NPEO 7 ). The adsorption to sewage sludge was related to the low-capacity, high-affinity sites observed for native sediment. Dependence of K d values on ethoxylate chain length was analyzed, in terms of both possible adsorption mechanisms, and the environmental fate and impact of NPEO x s as endocrine disruptors.