We present a combined experimental and computational density functional theory (DFT)/time-dependent DFT study of the geometry, electronic structure and optical absorption spectrum of the solar-cell sensitizers cis-[Ru(4,4'-COO--2,2'-bpy)(2)(X)(2)](4-) (X = NCS, Cl in water solution. The experimentally observed red-shift of the spectrum upon substitution of NCS- by Cl- is well reproduced by our theoretical model and appears to be related to stabilization of the t(2g-pi*) HOMOs in the NCS- complex. The pi* LUMOs of the Cl complex are destabilized by a smaller extent with respect to the HOMOs, due to the increased pi-back bonding interactions with the bipyridine ligands. (c) 2005 Elsevier B.V. All rights reserved.