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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 π* LUMOs as a result of the increased electron density on the deprotonated carboxylic groups. [Copyright &y& Elsevier]