The effect of pre-ozonation on the formation of four aldehydes (i.e. formaldehyde, acetaldehyde, glyoxal, methyl glyoxal) and two ketones (i.e. pyruvic acid and glioxalic acid) at the Beheshti water treatment plant in Hamadan, Iran were studied. Water samples were taken from the treatment plant at different points of the treatment train during September-January, 2013. The variation of formaldehyde, acetaldehyde, glyoxal, methyl glyoxal, pyruvic acid, and acid glioxalic concentration within treatment process were monitored in September and January. The results indicated that formaldehyde and acetaldehyde were the most abundant aldehyde species in the raw water. After pre-ozonation all of the aldehydes and ketones reached the maximum concentration. On the contrary, results show that the coagulation-filtration process was beneficial to the removal of aldehydes and ketones. Also, the results indicated that the level of aldehydes was increased after post chlorination. The percent increase was 52.3, 34.6, 12.1, 26.8 and 38% for formaldehyde, acetaldehyde, glyoxal, methyl glyoxal and glyoxalic acid, respectively, in September. Therefore, final chlorination led to increase in the formation of aldehydes in the pre-ozonated water. Ketones were not detected in the post-chlorinated water. Moreover, there was no correlation between total organic carbon (TOC) and total aldehyde and total ketone in water samples.