In-situ technique quartz crystal microbalance (QCM), differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were employed to investigate the effect of disodium ethylenediamine tetraacetate (EDTA) on photocatalytic reduction of mercury onto nanocrystalline titania (TiO2). Effects of EDTA on adsorption of Hg(II) and its photocatalytic reduction process at the surface of TiO2 in different pH solutions had been studied in detail. From the in-situ response to the adsorption of Hg(II) onto TiO2, the reaction rate and saturation adsorption amount were estimated about 4.71 x 10(-6)g mol(-1) min(-1) and 46.36 (mg Hg(II)/gTiO(2)) via the model of pseudo-second-order kinetics respectively. The photocatalytic reduction of Hg at the surface of TiO2 was influenced by pH and the mole ratio of Hg(II) to EDTA. When the ratio of Hg(II) to EDTA 1:1, it was most favorable for the photocatalytic reduction of mercury. In addition, the effects of HCOOH and EDTA on the reduction of Hg(II) was comparatively investigated and the mechanism on the photocatlytic reduction of mercury was illustrated. Therefore, it could be concluded that QCM. DPV and CV were effective methods for the investigation of photocatalytic reduction of complex heavy metal ions onto the surface of nanocrystalline TiO2. (C) 2010 Published by Elsevier Ltd.