Pulsed magnetic field gradient stimulated echo (PGSTE) NMR is used to investigate the simultaneous flow of two phases (an aqueous phase and an hydrocarbon phase) within a strongly water-wet sample of Fontainebleau sandstone. The Fontainebleau sandstone is prepared in increasing steady-state water saturations by a secondary imbibition process. The increase in the water saturation causes an increasing fraction of the oil phase (non-wetting phase) to become trapped within the sample. The stimulated echo dependence on the gradient pulse area, q, is used to derive the displacement probability, P(X), for a fixed observation time. These displacement probabilities clearly show the progressive trapping of the oil phase with increasing steady-state water saturations. Quantitative measurements of the fraction of the oil phase trapped were made from the echo attenuation function E-Delta(q), both as a function of water saturation and observation time. (C) 2004 Elsevier Ltd. All rights reserved.