The use of ATR-FTIR to probe the adsorption of oxalic and salicylic acids, and of mixtures of both, onto TiO2 (Degussa P-25) demonstrates the potential of the technique to characterise the evolution of the catalyst with time, including surface poisoning. Under equilibrium dark conditions, three surface species are formed by oxalate, and two by salicylate. Their stabilities, described by conditional Langmuir-type equilibria involving the dissociative, electroneutral adsorption of the acids H2L, are at pH 3.7, 2.4 x 10(6), 3.0 x 10(4) and 3.0 x 10(3) mol(-1) dm(3) for oxalic acid, and 2.9 x 10(5) and 9.1 x 10(3) mol(-1) dm(3) for salicylic acid. The nature of the species is discussed in terms of their spectral features. The displacement of each acid from the surface by addition of the other one was followed also by ATR-FTIR. The results demonstrate that oxalate displaces totally chernisorbed salicylate, whereas salicylate displaces oxalate only partially. These results are explained by assuming competitive chemisorption onto two different surface sites, plus oxalate adsorption onto a site that exhibits negligible affinity for salicylate. Irreversible adsorption by partially oxidised products in the course of photocatalytic processes can therefore be assessed by ATR-FTIR.