The photonic bands of various TiO2 photonic crystals filled with acetonitrile were investigated with the perspective for application to dye-sensitized solar cells. Finite-difference time-domain methods revealed that three-dimensional (3D) photonic crystals with diamond-log and inverse-diamond-log structures composed of TiO2 and an electrolyte had full photonic band gaps under certain conditions. The quality factor of the band gap and the electrolyte filling factor of the structures were optimized. Moreover, with the consideration for easy fabrication of such photonic crystals, two-dimensional (2D) photonic crystals, i.e., TiO2 slabs with square holes filled with electrolytes, were also investigated. Two-dimensional structures that have a full photonic band gap were also discovered. These discoveries may lead to early electrochemical applications of dye-sensitized photonic-crystal electrodes. (C) 2009 Elsevier Ltd. All rights reserved.