98 organic compounds were selected to investigate their trihalomethane formation characteristics. The selected organic compounds were chlorinated at 20°C in phosphate buffer solution (pH 7), and chloroform concentrations were measured after 24 hours. The results of this research are as follows : (1) Aliphatic 1, 3-diketones, acetyl compounds and citric acid form chloroform. (2) Aromatic 1, 3-diketones, acetophenones, phenols, anilines, methoxybenzenes and pyrrole compounds also form chloroform. The amount of chloroform formed from phenols is strongly affected by the type of isomers, that is meta-isomar forms higher amount of chloroform than ortho- and para-isomer. But this effect is not observed in the chlorination of acetophenones and anilines. (3) The chloroform yields from 1, 3-diketones, 1, 3-dihydroxybenzens, aminoacetophenone, hydroxyacetophenone and aminophenol are high. The chloroform yields from aromatic compounds are higher than those from aliphatic compounds. (4) It is suggested that chloroform yields from 1, 3- diketones and acetyl compounds are affected by enolization of these compounds. (5) 1-hydroxyethyl compounds is difficult to form chloroform.(6) Chlorine is consumed by oxidation or chlorination of organic compounds than by chloroform formation reaction.