This study investigates the adsorption and reactions of boron trichloride and its fragments (BCl x ) on the TiO 2 anatase (101) and rutile (110) surfaces by first-principles calculations. The results show that the possible absorbates on the TiO 2 anatase and rutile surfaces are very similar. The single- and double-site adsorption configurations are found for both anatase and rutile surfaces. The particular adsorbate feature on the anatase surface is its in-plane double-site adsorption by Ti and O from its sawtooth surface. The potential energy surface shows that BCl 3 can be adsorbed on the O site for both the anantase and rutile surfaces and the most of the BCl x reaction on both anatase and rutile surfaces are endothermic, except for the dissociative reaction on the rutile surface. The energy levels of the BCl x reactions between the anatase and rutile surfaces show that the rutile surface has lower energy levels than those of anatase surface. This result reveals that the BCl x dissociative adsorption more easily occurs on rutile surface than on anatase surface.