Merocyanine dyes that contain a carboxyl group and alkyl chain, like 3-carboxymethyl-5-[2-(3-alkyl-2-benzothiazolinyldene)ethylidene]-2-thiox o-4-thiazolidinone, enhance the efficiency of TiO2 solar cells. Knowledge of the bonding of this dye molecule to the rutile TiO2 surface will increase the understanding of the dye-semiconductor electron transfer responsible for the enhanced efficiency. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements and an ab initio core-excited state molecular orbital calculation for the merocyanine dye were performed. From these results, it was found that the main molecular plane forms a 47degrees angle with the surface normal, and the carboxyl group that bonds to the TiO2 surface is in alignment with the surface normal. STM experiments show that the dye molecules form aggregates on the TiO2, surface. (C) 2002 Elsevier Science B.V. All rights reserved.