Although the constitutive relationship of relative permeability as a function of saturation has been examined extensively in soil science and petroleum engineering, there is little information about this relationship for many systems encountered at hazardous waste sites. This lack of data has led to the use of estimation methods for relative permeability without verifying that they are indeed applicable to systems representative of hazardous waste sites. To assess the accuracy of methods commonly used for the prediction of two-phase relative permeabilities, drainage and imbibition relative permeabilities were measured for several organic liquid--water systems. A comparison of these measurements with estimates generated with five common methods showed that these methods are limited in their predictive capabilities. They estimated the relative permeability to the aqueous phase well, but failed to predict the low permeability to the organic liquid. For the relative permeability to the organic liquid to reach the predicted value of unity at the residual aqueous phase saturation, the effective permeabilities must be normalized by the permeability measured in the presence of residual water. The common estimation techniques make no provision for hysteresis. Since the aqueous phase showed little hysteresis, these estimations methods can be applied to both drainage and imbibition processes for this phase. However, the organic liquid phase showed hysteresis, and thus these techniques may only be appropriately applied to drainage for the nonwetting phase. ¿ American Geophysical Union 1993