Connate water is the name given to the water that occurs naturally in petroleum reservoirs in the form of an invisible dispersion or film within the oil phase. In this study, the effects of connate water on immiscible water/oil displacement processes in porous media were investigated. Special attention was directed towards the effects of connate water on viscous fingering instabilities and on oil recovery displacement efficiency. Experiments were performed using a two-dimensional, water-wet, consolidated porous medium cell of porosity 0.30. The fluid system employed was water displacing a lower-density paraffin oil phase. Three different flow modes were used for the experiments, namely, horizontal, vertical-upward and vertical-downward. For each flow mode, corresponding pairs of experiments were performed for five different flow rates in the absence and presence of connate water. At low flow rates, the irregularity of the viscous fingering pattern increased significantly in the presence of connate water and the corresponding oil recovery decreased. At high flow rates, the fingering pattern and oil recovery were largely independent of the flow mode and the presence of connate water. Depending on the flow mode situation, buoyancy effects can either stabilize or destabilize the displacement process when the densities of the two fluids are different.