The effect of carbohydrates on the swelling of the DMPC L(alpha) phase has been studied by small angle X-ray scattering (SAXS) and small angle neutron scattering(SANS). The carbohydrates are glucose, oligomers of glucose (up to n = 5), polymers of glucose (pullulan, M = 45 000), and a hydrophobically modified polysaccharide (cholesteryl-pullulan = CHP). Three regimes of swelling have been observed : (1) Small sugars (up to n = 3) penetrate into the aqueous layers of the lamellar phase and cause a monotonic swelling of the lamellar phase. (2) Macromolecular carbohydrates are sterically excluded from the aqueous layers and retain some of the water in a coexisting isotropic phase; this causes an osmotic compression of the lamellar phase. (3) Hydrophobically modified carbohydrates (CHP) can be introduced into the aqueous layers. The polymer containing lipid phase is a new lamellar phase (L(p)), with water thicknesses much larger than those observed in the absence of polymer.