Numerical models have been widely used to simulate multiphase flow in porous media for a variety of applications (e.g., NAPL migration in subsurface aquifers, carbon sequestration, agriculture, paper production, and petroleum reservoir development). The relationship between the difference in phase pressures and saturation is used as one of the important constitutive relationships in numerical models. Theoretical studies have suggested that this relationship should include a damping coefficient or capillarity coefficient () on the basis of thermodynamic considerations. A literature review suggests that the magnitude of this capillarity coefficient varies by over three orders of magnitude. While recent experimental studies have explored the effect of porous medium properties, effect of domain size, hysteresis, and the imposed boundary conditions on the magnitude of , there has been no experimental study investigating the impact of fluid viscosity on . This study reports on a series of primary drainage experiments conducted under both static and dynamic conditions in F70 silica sand. Fluid pairs used included water and silicone oil with two differing viscosities and slightly different densities (used as model nonaqueous phase liquids) in addition to air. Water saturation and both wetting and nonwetting phase pressures were measured in a custom-built aluminum column using EC-5 probes and tensiometers at three levels. Results show a strong dependence of the magnitude of the capillarity coefficient on effective fluid viscosity. This implies that consideration should be given for the inclusion of a capillarity coefficient in modeling tools used to simulate multiphase flow when fluids saturations are changing rapidly and when fluids have a large viscosity ratio.