The feasibility of the Photo-Fenton reaction with sunlight as irradiation source was tested with model waste waters (4-chlorophenol) in different weather conditions and at two sites. The degradation rate was found to decrease with rising length of path in experiments in a glass basin with a volume of up to 201 in Vienna. In a compound parabolic collector (CPC) at the Plataforma Solar de Almeria increasing the amount of the iron catalyst proved to accelerate the degradation while the degradation rate decreased with rising initial pollutant concentration.