The effects of pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content on sorption nonequilibrium during solute transport were evaluated. Nonequilibrium transport was observed to increase with pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content. Nonequilibrium transport of neutral organic compounds was not detected with low organic-carbon aquifer material, but was detected on higher organic sorbents from the unsaturated zone and the soil surface. For solute-sorbent combinations yielding retardation factors above 2, nonequilibrium during transport was observed. After experimentally accounting for slow solute diffusion in the aqueous phase and isotherm nonlinearity as potential contributors to nonequilibrium solute transport, sorption nonequilibrium was attributed to slow solute diffusion within the organic-carbon matrix. (Copyright (c) 1988 Elsevier Science Publishers B.V.)