Three-dimensional strong topological insulators (TIs) such as Bi 2 Se 3 , Bi 2 Te 3 and Bi 2 Sb 3 are intriguing for their surface metallicity in contrast to the insulating bulk. The metallic surface states of TI are known to be topologically protected and robust for impurities and disorders. In this work, we report first-principles calculations on the adsorption and dissociation of H 2 O molecules on Bi 2 Se 3 (111) to understand the chemical reactivity and the effect of oxidation on the surface metallicity. On the pristine (111) surface, H 2 O molecule is found to chemisorb on a subsurface Bi atom and form two additional hydrogen bonds with neighboring surface Se atoms. The adsorbed H 2 O molecule can be dissociated into a hydroxyl (OH) and H. The dissociated OH takes a surface Se site, and pushes up the Se atom that is bonded with the dissociated H. We examined a subsequent dissociation reaction of OH. The final reaction products are a substitutional O (O Se ) and an H 2 Se molecule floating in the vacuum. By examining the electronic structure, we found that the chemisorbed OH induces n-type doping. On the other hand, the O Se and the adsorbed H 2 O result in p-type doping. Throughout the whole chemical processes studied, the metallic surface state remains intact.