We have investigated the photocatalysis of partially deuterated methanol (CD(3)OH) and H(2)O on TiO(2)(110) at 400 nm using a newly developed photocatalysis apparatus in combination with theoretical calculations. Photocatalyzed products, CD(2)O on Ti(5c) sites, and H and D atoms on bridge-bonded oxygen (BBO) sites from CD(3)OH have been clearly detected, while no evidence of H(2)O photocatalysis was found. The experimental results show that dissociation of CD(3)OH on TiO(2)(110) occurs in a stepwise manner in which the O-H dissociation proceeds first and is then followed by C-D dissociation. Theoretical calculations indicate that the high reverse barrier to C-D recombination and the facile desorption of CD(2)O make photocatalytic methanol dissociation on TiO(2)(110) proceed efficiently. Theoretical results also reveal that the reverse reactions, i.e, O-H recombination after H(2)O photocatalytic dissociation on TiO(2)(110), may occur easily, thus inhibiting efficient photocatalytic water splitting.