A system in which a membrane separates two aqueous solutions of a non-electrolyte of di †erent but constant compositions is used to demonstrate a continuous transition from positive to negative osmosis and from positive to negative retention coefficients in Ðltration experiments. The membrane is a cation exchange membrane loaded with two deliberately chosen counterion species which compensate the electrical Ðxed charges of the membrane matrix. The parameter of the experiments is the mole fraction of one of the mobile counterion species in the pore Ñuid of the membrane. The counterion species which is preferential solvated by the non-electrolyte acts as a carrier for that component across the membrane. Increasing the mole fraction of the carrier ion species in the pore Ñuid causes two e †ects : it increases the transport rate of the non-electrolyte across the membrane as well as its coupling with the osmotic volume Ñow across the membrane. This leads to the stated changes of the transport properties of the membrane.