The primary aim of this study was to determine the effects of connate water on caustic flooding processes in porous media by employing acidified paraffin oil as the oil phase and aqueous sodium hydroxide as the water phase. The concentrations of the NaOH solutions were 2.5, 25.0 and 250.0 mM while the organic (linoleic) acid concentrations in heavy paraffin oil were 0.10 and 10.0 mM. Each displacement process was per-formed in the absence and presence of connate water at a displacement flow rate of 5.50 ml/h in a water-wet, consolidated porous medium. The results indicate that connate water has a greater effect on the displacement pattern for systems with higher interfacial tensions. For lower interfacial tensions, the patterns are similar both with and without connate water. At breakthrough, the presence of connate water in the cell generally increases the recovery, especially for those systems with lower acid concentration. After breakthrough, more oil was recovered from the systems containing connate water. Reducing the acid concentration has a large effect on the displacements, indicating that the organic acid is the limiting reagent. Finally, a general trend is that as the interfacial tension decreases, the number of fingers increases and the finger width decreases. (C) 2003 Elsevier Science B.V. All rights reserved.