Separation of acetic acid from water using silicalite filled PDMS membrane has been studied. The selectivity of the membrane for acetic acid has been found to increase with (i) increasing wt% of silicalite in the membrane and (ii) increasing hydrophobicity of the silicalite. The flux of acetic acid is however, found to behave in an opposite manner. An attempt has been made to propose a Dual Mode model for permeation in filled membranes and explain the results of this investigation as well as those available in the literature in the light of this model.