Dark adsorption and photocatalytic degradation of para-hydroxybenzoic acid (PHBA) was studied in the presence of Degussa P-25 titanium dioxide photocatalyst under concentrated solar radiation. This work includes the study of the effect of pH, and of the presence of different anions and cations on dark adsorption and photocatalytic degradation (PCD) of PHBA on titanium dioxide. The results obtained clearly indicate that there is a link between the extent of adsorption of PHBA and its PCD. Conditions that favor greater adsorption of PHBA also favor a greater PCD. This points to the possibility that the mechanism of PCD involves a surface reaction between adsorbed PHBA and OH ° radical. Anions were generally found to have a detrimental effect on the photocatalytic degradation of PHBA. Cl– caused the greatest decrease in the PCD followed by NO3 –, HCO3 – and SO4 2–. Cations Cu2+ and Fe2+ were not found to assist the degradation of PHBA, possibly due to the anions associated with them.