A new solid-state electrochromic system is presented. It is transparent and is comprised of a tungsten oxide and Prussian blue (PB) thin film couple in combination with a proton-conducting, solid polymer electrolyte. This electrochromic system exhibits rapid and deep optical switching; characteristic of a complementary configuration, both electrochromic films color and bleach in phase. Complementary electrochromic cells with the tungsten oxide-PB couple have previously been based on Li+ or K+-conducting electrolytes. A repetitively cycling cell has not previously been reported with a proton-conducting solid polymer electrolyte. The devices were operated at low applied voltages, +1.2 V to darken and -0.6 V to bleach. Repeated reduction and oxidation of the current system over 20,000 cycles has been demonstrated, indicating a large number of switchings without great degradation or irreversible side reactions. The sustained, high overall coloration efficiency of the devices suggests the insertion/extraction of protons into and out of both WO3 and PB films. The effects of cell size and operating temperature on the switching response are discussed.