As a typical smart material, ionic polymer metal composite (IPMC) has a sandwich structure, which consists of a base membrane and two thin metallic electrodes on both sides of the base membrane. The properties of the base membrane, Nafion as the most used base material, strongly affect the performance of IPMC actuator. This paper reports the effects of different additives, such as ethylene glycol (EG), dimethyl sulfoxide (DMSO), N, N-dimethyl formamide (DMF), and N-methyl formamide (NMF), on the performances of the casting membranes and SO-based IPMC actuators. Studies have shown that the microstructures of the casting membranes with EG and DMSO as additives are more loose and amorphous, leading to higher water contents and thus higher conductivity than those with DMF, NMF, and Nafion 117. Among the casting membrane-based IPMC actuators, EG-based IPMC actuator has larger deformation and blocking force, higher strain energy density and conversion efficiency at 2 V DC voltage, whose electromechanical properties are most close to that based on Nafion 117. POLYM. ENG. SCI., 54:818-830, 2014. (c) 2013 Society of Plastics Engineers