The permeability of some alcohols and aqueous solutions of ethanol through a poly(vinyl chloride) (PVC) membrane was investigated. The experimental results showed a large deviation from Darcy’s law. The deviation was attributed to the change in the physico-chemical properties of the fluid permeating through the pores. The capillary viscosity of a hydrophilic fluid passing through a hydrophobic medium was found to be much lower than its bulk value. It was also observed that for an aqueous ethanol solution, there existed a concentration interval in which the fluid passed through the PVC membrane without any change in its viscosity. Temperature and salt concentration affected differently the bulk and capillary viscosity. It was established that to validate Darcy’s law for all fluids-membrane systems, the viscosity has to be considered as a function of the permeate-membrane interaction, temperature, presence of salts and, probably, the tortuosity and sizes of the pores.