This paper describes the preparation of composite membranes comprising a poly[1-(trimethylsilyl)-1-propyne] (PTMSP) top-layer on a porous poly(acrylonitrile) (PAN) support. The PTMSP layer has different top-layer thickness in the range of 0.7–6.3 μm. The optimized PTMSP/PAN composite membranes with top-layer thickness of about 1 μm have ethanol permeability of 3.8 kg/(m2 h bar) and 90% retention of the negatively charged dye Remazol Brilliant Blue R (MW 626.5) at 5 bar. The permeability of methanol, ethanol or acetone through the PTMSP/PAN composite membranes is higher than a number of commercial available nanofiltration membranes, whereas all membranes have rejection of about 90% for negative charged dyes of different molecular weight. The permeability of methanol, ethanol and acetone through the PTMSP/PAN composite membranes depends on solvent viscosity and membrane swelling; the solvent viscosity seems to be the dominant factor.