Organic matter dissolved in seawater should be analyzed carefully since organic fouling is one of the main reasons for low performances of seawater reverse osmosis (SWRO) processes. Because of various ionic constitutions with high concentrations in seawater, the seawater organic matter analysis is difficult to handle with. Dissolved organic carbon (DOC) as a quantitative and general parameter and molecular weight distribution (MWD) as a qualitative and, more specific one for the same model organic matter, were measured with various ionic concentrations and constitutions. As a result, the wet oxidation method was found unreliable for measuring DOC in seawater. The ions inhibiting wet chemical oxidation are not only chloride but also other ions that are found in seawater. It was also found that MWD patterns for the same organic matter were variable along with different ionic concentrations. In conclusion, the uncertainty in seawater organic analysis exists because of various ionic inhibition effects, and it may induce wrong analysis results.