The photocatalytic removal of model humic and fulvic acids of different origins (terrestrial and aquatic) was investigated using TiO2 Degussa P-25 as the photocatalyst. The results were presented comparatively based on pseudo-first-order and the Langmuir-Hinshelwood kinetic models. The reaction rates calculated for the UV-vis parameters follow the general trend: IHSS soil humic acid > Aldrich humic acid > IHSS humic acid > Roth humic acid > IHSS fulvic acid. However, the rate constants show differences depending on the UV-vis absorption parameters. The evaluation of the Langmuir-Hinshelwood kinetic model reveals that, in all cases, Roth humic acid has the lowest Langmuir-Hinshelwood rate, rate constant and adsorption coefficient, and the ordering of the rates follows the same trend with that of pseudo-first order. Furthermore, data related to the molecular size distribution profiles of raw and treated humic acids suggest that the photocatalytic degradation occurs irrespective of the molecular size fractions.