The purpose of this work was to study the dependence of solar photocatalytic and solar water disinfection on solar irradiation conditions under natural sunlight. This dependency was evaluated for solar photocatalysis with TiO2 and solar-only disinfection of three microorganisms, a pure E. coli K-12 culture and two wild strains of the Fusarium genus, F. solani and F. anthophilum. Photocatalytic disinfection experiments were carried out with TiO2 supported on a paper matrix around concentric tubes, in compound parabolic collectors (CPCs) or with TiO2 as slurry in bottle reactors, under natural solar irradiation at the Plataforma Solar of Almeria (Spain). The experiments were performed with different illuminated reactor surfaces, in different seasons of the year, and under changing weather conditions (i.e., cloudy and sunny days). All results show that once the minimum solar dose has been received, the photocatalytic disinfection efficacy is not particularly enhanced by any further increase. The solar-only disinfection turned out to be more susceptible to changes in solar irradiation, and therefore, only took place at higher irradiation intensities. (c) 2007 Elsevier B.V. All rights reserved.