Development of inexpensive and flexible solid polymer electrolytes (SPEs) is critically important for the cost-effective electrochromic device (ECD) fabrication. The SPEs are also an integral part of the flexible ECDs which are projected to boost up the ECD's usability in several unexplored areas. Here we report the synthesis of polymethyl methacrylate (PMMA) based aprotic salt (LiClO4) containing SPEs, which are more or less electrochemically benign in the normal ECD operational voltage window. Experiments verified that the developed SPEs have good luminous clarity (transmittance > 92%), high ionic conductivity of the order of 10(-4) S/cm and excellent dimensional stability, all of which are required for the ideal ECD operations. Impedance spectroscopy showed an enhancement in the ionic conductivity and decrease in the solid electrolyte interface (SEI) resistance with increasing salt loading. Two different device configurations using these SPEs were fabricated and then subjected to detailed electrochemical studies. Comprehensive analysis established long term cyclic durability of the SPEs. Prototype ECDs fabricated using these SPEs rendered excellent optical modulation and coloration reversibility. (C) 2015 Elsevier B.V. All rights reserved.