We incorporated amphiphilic triblock copolymer into a lyotropic lamellar system and studied the effects of the hydrophilic chain length on the elastic nature of the lamellar membranes. The triblock copolymer consists of a hydrophobic chain (PPO) bounded by two identical hydrophilic chains (PEO). The hydrophobic chain anchors in the lamellar membranes and the hydrophilic chain decorates the membrane. We estimated the layer compression modulus by combining a small angle x‐ray scattering (SAXS) and a neutron spin echo (NSE) technique. The increase of the hydrophilic chain length changes the bending rigidity of membrane and enhances the repulsive forces between membranes. We discuss the results in terms of the entropic undulation interaction of lamellar membranes and comformational entropy of the hydrophilic chains. © 2004 American Institute of Physics