We present a combined density functional/time-dependent density functional study of the electronic structure and optical absorption spectrum of the charge-transfer sensitizer cis-[Ru(4,4'-COOH-2,2'-bpY)(2)(NCS)(2)] in water solution. To simulate the effect of different pH values, calculations have been performed for both the title complex and the corresponding tetra-deprotonated species, cis-[Ru(4,4'-COO-2,2'-bPY)(2)(NCS)(2)](4-). The experimentally observed blue-shift of the spectrum at high pH is well reproduced by our theoretical model and appears to be related to destabilization of the bipyridines pi(*) LUMOs as a result of the increased electron density on the deprotonated carboxylic groups. (C) 2004 Elsevier B.V. All rights reserved.