The intervesicular aggregation of phospholipid vesicles is induced by the addition of water-soluble polymers such as poly(ethylene glycol), dextran, etc. due to the interaction between the vesicular surface and the water-soluble polymers. The interaction can be expressed by the critical molecular weight (M(c)) of the water-soluble polymers for the aggregation of vesicles. The surface modification of vesicles with glycolipids (O-1,O-5-bis(octadecyl) N-maltooligonoyl-L-glutamate) accelerates the aggregation of vesicles induced by dextran; therefore, M(c) significantly decreased due to the surface modification. No dependence of phospholipid concentration and dextran concentration in an aqueous phase on the M(c) indicates that dextran does not act as a cross-linking agent among the vesicles. A clear dependence of the density of the saccharide chains on the vesicular surface on the M(c) suggests that dextran should adsorb on the surface of the vesicles by the interaction with the oligosaccharide chains on the surface and cause vesicular aggregation. A lower critical solution temperature was observed for this kind of interaction, and the critical temperature was controlled by changing the molecular weight of dextran.