Emulsions are encountered at different stages of oil production processes, often impacting many aspects of oilfield operations. Emulsions may form as oil and water come in contact inside the reservoir rock, valves, pumps, and other equipments. Snap-off is a possible mechanism to explain emulsion formation in two-phase flow in porous media. Quartz capillary tubes with a constriction (pore neck) served to analyze snap-off of long ("infinite") oil droplets as a function of capillary number and oil-water viscosity ratio. The flow of large oil drops through the constriction and the drop breakup process were visualized using an optical microscope. Snap-off occurrence was mapped as a function of flow parameters. High oil viscosity suppresses the breakup process, whereas snap-up was always observed at low dispersed-phase viscosity. At moderate viscosity oil/water ratio, snap-off was observed only at low capillary number. Mechanistic explanations based on competing forces in the liquid phases were proposed. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1993-1999, 2009