TiO2 modified with Cu2+ or Cr3+ ion (M-TiO2, M-grafted TiO2) was synthesized by a sol-gel method using Ti(OC3H7)(4) as a starting material or a graft method by impregnation of rutile TiO2. In the sol-gel method, the Brunauer-Emmett-Teller (BET) surface area of M-TiO2 was varied by preparing with or without dialysis of TiO2 sol (M-TiO2(D) or M-TiO2(ND), respectively). Even by using the sol-gel method, Cu2+ whose ionic size is larger than Ti4+ cannot be doped but is incorporated in TiO2 due to relatively weak interaction. Dissolution of Cu2+ ion was observed when Cu-TiO2(D), Cu-TiO2(ND) or Cu-grafted TiO2 was immersed into the acidic solution of 0.1 mol dm(-3) HCl. The photocatalytic activity for the degradation of 4-chlorophenol (4-CP) under visible light irradiation was the following order: Cu-grafted TiO2 > Cu-TiO2(ND) > Cu-TiO2(D). We have revealed that the photocatalytic activity of Cu-grafted TiO2 decreases linearly with an increase in the number-density of Cu2+ ion on the TiO2 surface. On the other hand, in the case of Cr3+ ion which has a similar size as Ti4+, the photocatalytic activity increased remarkably by doping Cr3+ in TiO2 by the sol-gel method. An increase in the BET surface area by conducting dialysis enhanced the photocatalytic activity. This paper demonstrates from the viewpoint of the higher photo catalytic activity as well as the chemical stability in acidic conditions that Cr-doped TiO2 prepared by the sol-gel method gives an advantage over Cu-grafted TiO2 for the degradation of 4-CP which is one of the non-degradable organic contaminants in water. (C) 2016 Elsevier B.V. All rights reserved.