To expand the still-rare studies about the effect of humic acids (HA) on the photocatalytic removal of water organic pollutants over TiO2, quinoline was selected, considering the previously investigated aromatics. Dark adsorption measurements have shown that the amount of quinoline (initial concentration (C-Q)(o) = 0.17 mmol L-1) adsorbed at equilibrium on TiO2 (50 m(2) g(-1); 2.5 g L-1) corresponds to only ca. 0.012 molecule nm(-2). By contrast, HA (Aldrich Na salt) was found to completely cover TiO2 for the initial mass concentration (C-HA(m)). = 81 ppm, assuming a flat structure for adsorbed HA and a mean molar mass of 4 kg. Notwithstanding, HA did not hinder quinoline adsorption. Without TiO2, the fraction of quinoline chemically bound to HA was only ca. 1.5% for (C-HA(m))(o) = 87 ppm; however, a weak quenching of quinoline fluorescence by HA illustrated the existence of looser interactions. The photocatalytic (lambda greater than or equal to 340 nm) initial removal rate, r(o), of quinoline ((C-Q)(o) = 0.077 - 0.77 mmol L-1) without HA was in agreement with a pseudo-first-order Langmuir-Hinshelwood kinetics, however, with an adsorption constant much higher than that deduced from dark adsorption. This difference suggests that the quinoline molecules involved should be not only those present in the monolayer, in accord with previous considerations for other poorly adsorbed compounds (J. Cunningham et al. In Aquatic and Surface Photochemistry; Helz, G. R. et al., Eds.; Lewis Publishers: Boca Raton, FL, 1994, p 317). For (C-HA(m))(o) greater than or equal to 12 ppm, r(o) was also pseudo-first-order at least initially, and its decrease was consistent with HA UV-light absorption (inner-filter effect) more than with the competition for the active species between HA and quinoline. For (C-HA(m))(o) = 6 ppm, r(o) was slightly, but significantly, higher than without HA. This net, as yet unreported, favorable effect of HA, which was counterbalanced at higher (C-HA(m))(o) /[TiO2] ratios, was shown not to arise from HA photoexcitation and is tentatively suggested to be caused by the sequestration of a fraction of quinoline molecules close to the TiO2 surface owing to adsorbed HA.