The effects of particle size (5, 35 and 70 nm) on the sorption of myo-inositol hexakisphosphate (IHP) and inorganic phosphate (KH2PO4, Pi) on gamma-Al2O3 nanoparticles were investigated using batch sorption experiments, zeta potential measurements and solid-state nuclear magnetic resonance spectroscopy (NMR). The results show that the maximum sorption densities (mu mol m(-2)) for IHP and Pi increase with decreasing gamma-Al2O3 particle size. The sorption affinity of gamma-Al2O3 for IHP and Pi generally increases with decreasing particle size, and the sorption affinity for IHP is approximately one order of magnitude greater than that for Pi. In our experimental time scale, surface complexation is the main mechanism for IHP and Pi sorption on large size gamma-Al2O3. While an additional surface precipitation mechanism, indicated by solid-state P-31 and Al-27 NMR data, is partly responsible for the greater sorption density on very small size gamma-Al2O3. Compared with Pi, the effect of particle size on the sorption of IHP is more pronounced. The results suggest a size-dependent surface reactivity of Al2O3 nanoparticles with Pi/IHP. The underlying mechanism will also be relevant for other small nanosize (hydr)oxide particles and is important for our understanding of the role of small nanoparticles in controlling the mobility and fate of organic and inorganic phosphates in the environment. (C) 2015 Elsevier Inc. All rights reserved.