In this work, we precise and discuss the adsorption isotherms in aqueous phase of various substituted phenolic compounds and organic diacids, and we characterize the adsorbates on TiO2 using infrared spectroscopy in diffuse reflectance mode (DRIFT). We have brought to the fore by DRIFT that chemisorption of an organic acid proceeds very fast via titanium carboxylate formation. On the contrary, phenols are poorly adsorbed and lead to Ti-phenolate adsorbates. The adsorption isotherms of all compounds have been determined experimentally. In the case of phenols, at low concentrations, the Langmuir equation is followed. Increasing concentration leads to a complex situation and multilayer adsorption has to be invoked as suggested by several authors. The adsorption of phenolic compounds is governed by several parameters and inductive or mesomeric effects of the substituants alone cannot explain the complex-invoked mechanisms.