The relationship between phase behaviour and mobilisation of residual oil using Bentheimer cores has been investigated for the system: water -n-decane - nonionic surfactant (C12E5). A part of the phase prism for the system (temperature as the z-coordinate) has been determined. Adding 500 ppm polymer increases the system hydrophobicity corresponding to approximately 5 degrees C. Recoveries of residual oil after waterflooding by surfactant flooding, oil cuts, pressure-drop profiles, and endpoint relative permeabilities have been determined for 17 experiments. In addition, determination of the surfactant retentions has been performed for eight of these flooding experiments. The flooding experiments were conducted varying the phase behaviour, flood rate, and wettability conditions of the cores. For water-wet cores, it is shown that mobilisation of the residual oil is correlated to the interfacial tension and the capillary number, and that it is independent of II-, III and II+ phase behaviour. In mixed-wet cores, the highest recovery was found when flooding with a system having II+ phase behaviour. It is probable that solubilisation of the surfactant in the residual oil and a film displacement mechanism caused the high recovery in the mixed-wet cores at II+ phase behaviour.