Heterogeneous photocatalytic degradation of many organic pollutants, such as phenol and phenol derivatives, may be optimised if the catalyst surface saturation and the appearance and accumulation of non-photocatalytically degradable intermediates is avoided. It has been shown that under certain concentration threshold the highest degradation efficiencies are achieved. Over these concentrations, degradation rates become constant owing to the limited catalyst surface. By the dosage of the contaminant, currently in an aqueous solution, the process may be optimised, thus avoiding the formation of inert intermediates which may be more toxic than the parental compound. The effect of dosage on the photocatalytic degradation of phenol and phenol derivatives, such as salicylic acid and 4-aminophenol has been studied. Comparatively notably higher efficiencies have been obtained compared to those of the high initial single dose experiments (non-dosage), for which high initial concentrations of the organics resulted in the catalysts poisoning. Degussa P-25 and its combination with 13% (w/w) activated carbon, namely AC−TiO2, have been used as catalysts. Almost complete degradations are achieved at low dosage rates (1–2 pmm/min). At higher dosage rates, different processes such as catalyst poisoning predominate, resulting in lower degradation efficiencies.