Applicability of TOC and COD values was examined as an indicator of organic contamination levels of subsurface brines from water-soluble natural gas fields. It was found that those brines have unique chemical characteristics as shown bellow, comparing with those seawaters.1) In TOC measurement, elimination of inorganic carbon by PH lowering made higher reproducibility and lower TOC values than those obtained by conventional methods. It was due to dissolved methane in those brines.2) In COD measurement, organic compounds in those brines were more easily oxidized than those in seawaters.3) The TOC values of those brines have strong correlation with the COD values. However, there was no such correlation in regard to seawaters.As mentioned above, it can be concluded that the TOC value is a more appropriate indicator of organic contamination levels of subsurface brines from water-soluble natural gas fields than the COD value.