Quantum yields of the photocatalytic DCA degradation in aqueous titanium dioxide suspensions (Hombikat UV100, Aeroxide P25) were determined employing a black body like photoreactor. The amounts of photons absorbed by the photocatalysts per unit time were determined by chemical actinometry varying the photon flux and the photon flux density. The photocatalytic DCA degradation experiments were performed under zero order conditions regarding the concentration of the probe compound. The obtained results suggest that the quantum yield of the photocatalytic DCA degradation depends on the photon flux density. Only the low flux density resulting from a large surface area of the light inlet seems to allow the determination of a quantum yield as a photocatalyst-inherent property.