Real-time monitoring of photocatalytic methanol decomposition over anodized iron oxide nanotube arrays (FNAs) in high vacuum was performed. Switching phenomena in the partial pressures of H-2, CO and H2O were observed simultaneously according to the ON/OFF sequence of visible and ultraviolet light illuminations over Pt-loaded FNAs prepared with photodeposition, while no change in these gas species appeared for bare FNAs and iron foils. The absence of formaldehyde (CH2O) in the intermediates during methanol photodecomposition suggests that Pt-loaded FNAs can possibly work as a catalyst for oxidation of CH2O. The photoreforming of methanol to hydrogen without any external bias in Pt-loaded FNAs presumably indicate some differences in photocatalytic reaction mechanisms between in liquid and in high vacuum. The dependence of the Pt photodepositon time on gas phase photocatalysis suggests that the selective deposition of Pt cocatalyst particles on the aperture parts of the nanotubes is necessary for efficient gas phase photoreforming.