The initial rate of electron storage was estimated from a reflectance change (at 600 nm) of the film, because the reflectance decreases as WO3 is reduced from pale yellow WO3 to blue MxWO3 (Eq. (2)). The dependence of the initial decrease rate of reflectance, [−d(reflectance)/dt]0, on the W/Ti mole ratio and the light intensity is shown in Fig. 3(a). The initial rates of electron storage were evaluated from these values on the basis of the relationship between the reflectance change and the amount of charge stored. The amount of charge stored was measured by constant current density re-oxidation of the reduced film at 3 μA cm−2 (cut-off potential = −0.1 V vs. Ag∣AgCl∣KCl(sat)). The typical discharging curve is shown in Fig. 3(c). The relationship between the reflectance change and the amount of charge looks like the curve of the inset in Fig. 3(a). The apparent quantum yield thus estimated (from Eq. (4)) is plotted against the W/Ti ratio in Fig. 3(b).