Recently many complaints of the musty odor of city water are annually increasing in districts where source water is taken from lakes and it confirmed that the odor is mainly caused by the musty substances, such as Geosmin and 2-methylisoborneol (2-MIB), which are produced by the metabolism of algaes in the water. But usually, the analysis of them is not so easy, because the concentration of them is extremely low in the water.
In this paper, a simple extraction method of Geosmin and 2-MIB was examined by using the adsorption with activated charcoal and the extraction with methylene-chloride as the privious treatment of samples on the GC-MS analysis and the degree of sensory odor of city water was measured by applying the tri-angle test. And also, the influence of residual chlorine in the water for the odor of city water was considered. The results obtained are as follows.
1) The minimum detectable limit was 3 ng for Geosmin and 1 ng for 2-MIB in this analytical method, and the coefficient of variation of obtained values was within 10%.
2) The recovery percentage on this analytical method was unfavorable, as only 40%, for 10 ng of Geosmin. It seems that the desorption of substances which captured on the activated charcoal was unexpectedly incomplete.
3) It was detected that the concentration of Geosmin in the city water was 0.183 ug/l for Otsu and 0.04 ug/l for Kyoto. And also, the values of D/T (odor concentration) for city water at 20°C was 18 holds for Otsu and 20 holds for Kyoto.
4) It was recognized that the degree of sensory odor intensity of city water is considerably influenced by the odor of residual chlorine in the water, and the value of D/T caused by residual chlorine in the water reached to 14 holds for Otsu and 19 holds for Kyoto.

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