The exposure of water to sunlight radiation improves the microbiological quality of water. This treatment process called solar water disinfection could be used at household level to treat small quantities of water for drinking purposes. The bactericidal effect of UV-A and violet light and their combined effects assumed as hypothesis by earlier research could be verified by the laboratory tests carried out at EAWAG. A 3-log reduction of E. coli requires a fluence close to 2,000 kJ/m² or 555 W·h/m² (dose of solar radiation integrated in the 350-450 nm wavelength range), which correspond to ~5 h of mid-latitude midday summer sunshine. The same dose inactivates also the bacteriophage f2 and a rotavirus to a similar order of magnitude, whereas a picornavirus (encephalomyocarditis virus) was observed to be twice as resistant. Water temperatures with a threshold of ~50 °C considerably increase the inactivation rate of bacteria induced by solar radiation whereas the inactivation rate of viruses steadily increases with temperatures in the range of 20-50 °C. The recorded synergetic effects of solar radiation and thermal water treatment favour a combined use of these two water-treatment processes.