Complementary tungsten oxide (WO3)-Prussian blue (PB) electrochromic devices (ECDs), in combination with Li+, K+ or H+-based electrolytes, have been proposed by many researchers for solar attenuation and glare reduction applications. In this study, cycling and at-rest stabilities of a solid-state ECD comprised of a WO, and PB thin film couple with a proton-conducting solid polymer electrolyte are discussed. The transmittance of the device varied from 71.5% to 6.0% at 550 nm. The device was darkened or bleached by the application of + 1.2 V or -0.6 V, respectively. Repeated switching or cycling of the ECD over 20,000 cycles has been demonstrated, indicating a large number of switchings without great degradation or irreversible side reactions. Nevertheless, at-rest stability in the absence of cycling, at room temperature over months, has not been achieved. The cause of at-rest instability during darkened-state or bleached-state storage will be discussed. Issues related to the scale-up are also discussed.