The effect of the composition of the copolymer of 1-carboxy-N,N-dimethyl-N-(2-methacryloyloxyethyl) methanaminium inner salt (CMB) and n-butylmethacrylate (BMA) (poly(CMB-co-BMA)) as a surface modification polymer on the low-fouling properties of dynamically formed membranes was systematically investigated, and the optimum composition for this method was found to be 35:65 [mol:mol], which was prepared from the monomer solution containing 50:50 [mol:mol] of CMB:BMA in the feed (CMB50). This optimized dynamically formed membrane exhibited excellent low-fouling property against both hydrophilic protein and hydrophobic protein. To discuss the reason for this low-fouling property, the hydration states of the polyCMB-water and CMB50-water systems were measured using DSC. The results showed that the water structure in the polyCMB-water system is very similar to that in the poly(2-methacryloyloxyethyl phosphorylcholine) (polyMPC)-water system, which is one of the best-known zwitterionic polymers used for biomaterials, and that the water structure in the CMB50-water system has much in common with that in the polyCMB-water system. These water structures are one of the reasons for this system's low-fouling properties. (C) 2013 Elsevier B.V. All rights reserved.