A reversible color change of WO3 has been widely studied to develop new energy-saving technologies such as smart windows, rewritable paper, and information displays. A blue coloration arises from the intervalence charge transfer between W(VI) and W(V), which is partially formed by the reduction of WO3 under UV light or an applied voltage. This means that WO3 has a mixed-valence state of W(V) and W(VI) upon the reduction. However, despite many studies for various applications, how many W(V) atoms are formed and contribute to the intervalence charge transfer (IVCT) transition remains unclear because W(V) formed in WO3 cannot be determined quantitatively. We determined the amount of the photogenerated W(V) in an aqueous WO3 colloidal solution containing ethylene glycol (EG) by observing the localized surface plasmon resonance (LSPR) peaks of Ag nanoparticles which were produced by a redox reaction between W(V) and Ag+. EG acted as a hole scavenger to suppress the recombination between the photogenerated holes and electrons. First, we explored the reaction condition where only the IVCT transition was observed under UV irradiation, and then it decreased in response to the increase in the LSPR peak in the dark. Under such a condition, the absorbance at 775 nm (A775) due to the IVCT transition was observed after the UV irradiation for 30 s, and the absorbance at 410 nm (A410) due to the LSPR absorption was obtained when A775 completely disappeared in the dark. Experiments were performed at various UV intensities to confirm a proportional relationship between A775 and A410. Electron spin resonance measurements revealed that A775 was proportional to the amount of W(V). Furthermore, Ag nanoparticles were synthesized by a polyol reduction method to obtain the relationship between the LSPR peak intensity and the Ag+ concentration, which was consumed for the formation of Ag. On the basis of all of these relationships, A775 of 1.669 corresponded to 2.53 × 10-4 mol dm-3 W(V), which was estimated to be only 0.21% of 0.12 mol dm-3 WO3 used in this study, and the molar absorption coefficient for the IVCT transition between W(V) and W(VI) was evaluated to be 6.85 × 103 dm3 mol-1 cm-1.