Plasticized liquid membranes and processes for separating monosaccharides from disaccharides and from other monosaccharides employing the membranes are provided. Fructose is separated from glucose via the membrane process to provide fructose in concentrations sufficient for HFS (high fructose syrup) and sucrose, glucose and fructose are separated from crude sources such as molasses, sugar cane juice and beet sugar juice. The plasticized membranes are prepared by dissolving a lipophilic polymer such as cellulose triacetate, an organic liquid plasticizer such as a hydrophobic ether compound e.g. 2-nitrophenyloctyl ether and, a carrier compound such as a quaternary ammonium salt or a boronic acid compound with lipophilic substitution in a suitable solvent which upon evaporation provides the plasticized membrane. The membrane produced is homogeneous having all three components equally dispersed in the membrane. Also provided are new boronic acid compounds which are especially useful carrier compounds for the transport of monosaccharides such as fructose through the membranes.