The lateral distribution of Ru(4,4′-dicarboxy-2,2′-bipyridine) 2 (NCS) 2 (N3) dye molecules on a titanium dioxide (TiO 2 ) surface was examined by using a scanning tunneling microscope. Pivalate ((CH 3 ) 3 CCOO − ) -covered rutile TiO 2 (110) surfaces were immersed in acetonitrile containing the N3 dye. The N3 molecules which replaced the pivalates were observed as protrusions embedded in the pivalate monolayer. A two-dimensional radial distribution function indicated that the adsorbed N3 molecules tended to be aggregated. Trapping of an N3 molecule in the solution by a preadsorbed N3 molecule was proposed as a driving force for the aggregation. The hydrogen bonds between the carboxyl groups of two N3 molecules contribute to the trapping.