Abstract: We have performed a surface photocatalysis study of the molecularly adsorbed forms of methanol on a vacuum-annealed rutile TiO₂(110)-(1×1) surface using temperature-programmed desorption both with and without coadsorbed water to investigate the effect of water and methanol on the photocatalytic dissociation of each other on rutile TiO₂(110)-(1×1). Our experimental results show conclusively that methanol has no effect on photocatalytic water splitting with water covered rutile TiO₂(110)-(1×1) surface. Further experimental results also show that water does not affect the process of photocatalytic methanol dissociation but does suppress photo-induced desorption of the formaldehyde product of methanol photocatalysis and can enhance the formation of the methyl formate product.