The inefficiency of solar disinfection (SODIS) for the disinfection purposes can be improved by concentrating the sunlight i.e. by using solar collector disinfection (SOCO-DIS) system as investigated by authors in previous research or by increasing the exposure time of the targeted water to direct sunlight. Under weak sunlight conditions, however, this still remained ineffective as highlighted by earlier research. The efficiency of solar-based disinfection systems for the treatment of stored rainwater is further improved based on the better performance of these systems at low pH. by adding commonly available and inexpensive food preservative products, such as lemon and vinegar. Lemon and vinegar both increased the disinfection efficiency in SODIS by about 40% and completely disinfected rainwater in a SOCO-DIS system under weak weather conditions by lowering the pH to 3. An optimum combination of 2.5 ml (0.25%) of lemon and 1.7 ml (0.17%) of vinegar was selected to avoid any taste or odor problems while maintaining complete disinfection. Results showed that the choice of catalyst (lemon or vinegar in this study) was also an important factor in addition to low pH for disinfection using sunlight. (C) 2011 Elsevier B.V. All rights reserved.