A new passive solar water pasteurization system based on density difference flow principles has been designed, built and tested. The system contains no valves and regulates flow based on the density difference between two columns of water. The new system eliminates boiling problems encountered in previous designs. Boiling is undesirable because it may contaminate treated water. The system with a total absorber area of 0.45 m(2) has achieved a peak flow rate of 19.3 kg/h of treated water. Experiments with the prototype systems presented in this paper show that density driven systems are an attractive option to existing solar water pasteurization approaches. (c) 2004 Elsevier Ltd. All rights reserved.