The main purpose of this work is to research the solvent composition effect on the streaming current of porous plug formed by monodisperse polystyrene microspheres. Polystyrene was selected as the disperse material because monodisperse, spherical polystyrene latexes have proved to be useful model systems, whose surface charge density can be conveniently controlled. A polystyrene latex was prepared by emulsifier-free emulsion polymerization, having a particle size of 1.2 mu m and a surface charge density of -34.0 +/- 1.0 mC/m(2). Binary mixtures of ethanol-and l-propanol-water were used to understand the effect of the liquid phase composition on the streaming current of polystyrene porous plugs. For the ethanol-and l-propanol-water mixtures the streaming current depends nonlinearly on the molar alcohol fraction, This nonlinear dependence can be explained by the variation of the viscosity and dielectric permittivity of the liquid with solvent composition. The zeta-potential plays also an important role in the explanation of the above dependence, especially with binary mixtures low in alcohol where a minimum in the zeta-potential appears. The electrokinetic results obtained in this work suggest that the basically hydrophobic polystyrene surface is modified by the presence of hydrophilic sites. Adsorption of alcohol molecules on the charged hydrophilic sites leads to desorption of counterions from the inner part of the electrical double layer.