Molecular contaminants, outgassed from organic materials such as paints, glues, and cables, degrade the optical properties of spacecraft. In our previous work, we reported on the photocatalytic activity of TiO2, a fundamental photocatalyst material, in a vacuum, utilizing a quartz crystal microbalance (QCM) technique for space applications aimed at decomposing molecular contaminants. The results revealed that TiO2/QCM successfully removed contaminants, achieving a maximum mass removal of approximately 2 μg/cm^2 in vacuum. In this study, we investigated the photocatalytic activity of MoO3 in vacuum, aiming to achieve higher performance than that of TiO2. The results indicated that MoO3 was more effective in removing contaminants than TiO2 in vacuum.