Formaldehyde (FA) and FA formed by boiling are metabolic products and biodegradable (Yamada et al., 1991). However, it is important to understand the potential amount of FA in drinking water. Laboratory studies were conducted from February, 1989 to May 1990 to evaluate the effects of ozonation on the formation of FA in surface water. FA in chlorinated tap waters also was analyzed. FA in regular tap water and the FA in tap water which was boiled ranged from 0 to 25.8 μg/L and from 18.0 to 73.5 μg/L, respectively. In surface water, the concentrations of FA and FA formed by boiling ranged from 0 to 24.0 μg/L and from 1.5 to 88.0 μg/L, respectively. After ozonation (ozone at 5 mg O3/L applied for 5 min) of the surface water, the range increased from 10.0 – 110 μg/L and 21.0 - 243 μg/L, respectively. The relationship between TOC and the amount of formed was as follows: