A series of BiOBr nanosheets was hydrothermally synthesized through pH adjustment of precursor suspensions. X-ray diffraction, X-ray photoelectron spectroscopy, electronic microscopy and UV-vis diffuse reflectance spectroscopy analyses indicate that these nanosheets possess similar crystallinity, surficial composition, structures, band gaps and highly exposed (0 1 0) facets, but different lateral sizes and thicknesses. The visible-light photoactivity of these nanosheets in photodegradation of salicylic acid increases with reduced nanosheet thickness. An inversely linear relationship was obtained between the pseudo-first-order rate constant (k) and the nanosheet thickness. The photoactivity enhancement of the samples with reduced nanosheet thickness is ascribed to the decreasing recombination efficiency of photogenerated charge carriers. This study indicates that the photoactivity of the BiOBr nanosheets with dominant exposed (0 1 0) facets decisively depend on the nanosheet thickness, rather than the percentage of exposed active facets ((0 0 1) facets), which may be universal for other two-dimensional photocatalysts. (C) 2015 Elsevier B.V. All rights reserved.