The membrane is the key component of a redox flow system, as it has to avoid cross-contamination of the ions of the active material as well as to permit high proton conductivity. A membrane with drastically reduced cross-contamination ability would overcome the main problem of several redox couples and, in consequence, a wider variety of redox couples would potentially allow redox flow systems with higher energy densities or lower material costs. This work deals with modifications of Nafion membranes to decrease the cross-contamination ability without affecting the proton conductivity. Several in situ gel reactions were used for modification of the membranes. Cross-contamination has been reduced in this work by a factor of 2 compared to previous works by Xi et al. [J. Power Sources, 166, 531 (2007)] and Teng et al. [J. Power Sources, 189, 1240 (2009)]. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3456625] All rights reserved.