Humic substances (HS) are formed during the degradation of plant and animal material, and both microbiological and abiotic processes contribute to their production. They impart a brown/yellow colour to water and can bind with metals and organic pollutants, affecting their mobility and bioavailability, and are precursors of mutagenic halogenated compounds in water formed after chlorination. Treatment of drinking water by UV irradiation is a successful disinfection technique. Passive systems for disinfection of drinking water based on minimal energy and materials consumption and exploiting only the available solar energy can be an alternative, mainly for remote and dispersed communities. This work presents results on the photolytic and photocatalytic (with TiO2) alterations of humic substances monitored in a UV254 (nm) reactor and a Solar Cocentric Parabolic Concentrator (CPC) reactor for drinking water treatment. For the UV254 (nm) reactor, photolysis of humic substances accounts for 80-90% and complete mineralization is achieved by the addition of small amounts of catalyst. For the CPC reactor, photolysis accounts for 60-70%. The reduction of humic substances initial concentrations may reach 80-90% for the runs of the reactor with immobilized catalyst and values of energy accumulated of 2 MJ m(-2) L-1.