Molybdenum trioxide sol and gel were prepared via reaction metal Mo powder with peroxo-hydrate solution and they could turn into blue color when irradiated by high pressure mercury lamp. Raman spectra show that the structrue of MoO3 sol-gel could be rearranged showing short-range order after UV irradiation. MoO3 sol-gel in ethanol could show photochromism by UV irradiation even only exposing in solar light for a long time. Electron spin resonance (ESR) and X-ray photoelectron spectrum (XPS) measurements at room temperatrue show MoO3 could form a blue colored bronze, and Mo^6^+ could be reduced to Mo^5^+ by light excitatio detected in photocolored gel in air.

Key words: MOLYBDENUM TRIOXIDE, SOL-GEL PROCESS, PHOTOCHROMISM, X- RAY PHOTOELECTRON SPECTROMETRY, ELECTRON SPIN RESONANCE