A new type of cation-exchange nanocomposite membranes was prepared by in-situ formation of ZnO nanoparticles in a blend containing sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) and sulfonated polyvinylchloride via a simple one-step chemical method. As-synthesized nanocomposite membranes were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction. The SEM images showed that ZnO nanoparticles were uniformly dispersed throughout the polymeric matrices. The effect of additive loading on physicochemical and electrochemical properties of prepared cationexchange nanocomposite membranes was studied. Various characterizations revealed that the incorporation of different amounts of ZnO nanoparticles into the basic membrane structure had a significant influence on the membrane performance and could improve the electrochemical properties.