For the understanding of the activity of TiO₂ photocatalysts, knowledge of the activities of different crystal facets is necessary. This information can be achieved by the investigation of well-defined single-crystalline TiO₂ surfaces. In this study, the photocatalytic activity of different anatase, brookite, and rutile single-crystal wafers with only one exposed surface has been investigated via the oxidation of methanol and the hydroxylation of terephthalic acid, respectively. X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements have shown that all surfaces are clearly defined and possess a smooth surface, which allows a reliable comparison of the photocatalytic activities. The investigated anatase surfaces show higher activity than the rutile surfaces, while the brookite surface is interestingly the least active one. To the best of our knowledge, there are no other reports based on the investigation and comparison of well-defined TiO₂ anatase (100), anatase (001), and brookite (100) single-crystalline surfaces concerning their photocatalytic activity. Furthermore, the influence of the coordination of the superficial titanium and the oxygen ions on the photocatalytic activity is discussed.