It is demonstrated how NMR imaging can be used noninvasively to quantify the volume of nonaqueous phase liquid (NAPL) present in an otherwise water-saturated porous medium. Data were recorded during the operation of a pump and treat (PT) remediation scheme on a bed of water-saturated sand contained within a vertical column. The model contaminant used was n-hexanol, and three different aqueous flow rates were employed. These data were then critically compared with the predictions of three models currently used to describe the dissolution of NAPL during a PT scheme : the linear mass-transfer model, the pore-diffusion model, and the shrinking-core model. The pore-diffusion model gave best agreement with the experimental data. However, none of the models predicted the observed dependence of mass transfer on Darcy velocity.