The levels of formaldehyde and acetaldehyde in bottled drinking water and the effect of storage time and storage conditions on their levels were determined. A total of 144 samples of six brands of bottled drinking water were purchased from Dar es Salaam, Iringa, Mwanza and Arusha regions in Tanzania. Analysis was performed using HPLC on the 1st day and after 14 and 42 days of storage in a refrigerator, at room temperature and exposure to sunlight. The levels of formaldehyde and acetaldehyde in the refrigerated samples ranged from 2.46 to 19.25 μg/L and from not detected (ND) to 58.70 μg/L, respectively. In samples stored at room temperature, the levels of formaldehyde and acetaldehyde were up to 23.26 μg/L and 36.10 μg/L, respectively. The highest levels of formaldehyde and acetaldehyde in samples exposed to sunlight were 46.0 μg/L and 187.0 μg/L, respectively. The lowest levels of the aldehydes were found in refrigerated samples and the highest levels were in samples exposed to sunlight. The levels of these aldehydes increased with increased storage time. The levels were below the WHO acceptable limits; however, the findings indicate that the levels could be elevated with increased storage time and exposure to sunlight.Ministry of Education and Vocational Training and Sokoine University of Agriculture