The binding of catechol derivatives (LH2 = catechol, 4-methyl catechol, 4-t-butyl catechol, and dopamine) to 1- and 4.7-nm TiO2 nanoparticles in aqueous, pH 3.5 suspensions has been characterized by UV-vis spectroscopy. The binding constants derived from Benesi-Hildebrand plots are (2-4) x 10(3) M-1 for the 1-nm nanoparticles and (0.4-1) x 10(4) M-1 for the 4.7-nm particles. (TiL3)-L-IV complexes were prepared from the same catechols. The L = methyl catechol, and dopamine complexes are reported for the first time. The TiL3 reduction potentials are not very sensitive to the nature of the catechol nor evidently are the binding constants to TiO2 nanoparticles. The intense (epsilon >= 10(3) M-1 cm(-1)), about 400-nm, ligand-to-metal charge-transfer (LMCT) absorptions of the nanoparticle complexes are compared with those of the TiL3 complexes (epsilon 10(4) M-1 cm(-1)) which lie in the same spectral region. The nanoparticle colors are attributed (as are the colors of the (TiL3)-L-IV complexes) to the tails of the about 400-nm LMCT bands.