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During the last decade intensive research work has been carried out in the field of photocatalysis because upon UV illumination photocatalysts such as titanium dioxide (TiO2) are able to catalyze the degradation of organic compounds in waste water as well as in air. However, the reaction mechanisms and the limitations of the photocatalytic processes have been studied in the literature less frequently, although the understanding of the fundamental processes is essential in order to design a photocatalyst with an improved photocatalytic activity. In this work the effect of O2 and H2O on the adsorption and the photocatalytic degradation of gaseous acetaldehyde over TiO2 has been investigated in the dark and upon UV illumination. The surface processes have been elucidated by means of in situ ATR-FTIR spectroscopy, while the gas phase was analysed by GC/PID and MS. The results obtained from the surface and from the gas phase were correlated with each other in order to gain a deeper insight into the adsorption and the degradation mechanism of acetaldehyde. In this study it is shown, that in the dark the TiO2 surface is able to catalyze an aldol condensation of two adsorbed acetaldehyde molecules forming crotonaldehyde. These surface processes are strongly affected by the humidity, because an increasing water vapor content induces a decrease in the adsorption of acetaldehyde molecules resulting in a reduction of the crotonaldehyde formation. In contrast, molecular O2 does not exhibit an influence on the surface processes in the dark. However, upon UV illumination the degradation of acetaldehyde is strongly affected by the O2 concentration, because acetaldehyde is incompletely degraded when O2 is not present. Besides the important role of O2, isotopic studies using Ti18O2 show that lattice oxygen atoms from the TiO2 surface are involved in the photocatalytic degradation process of acetaldehyde in the absence of O2.