Idod (Immediate Dissolved Oxygen Demand) is a value of the oxygen demand after 15 minute of inoculation by inorganic reducing salts. Industrial development and urban enlargement are bringing water pollution deeply, and industrial waste waters are the source of the inorganic reducing salts. Author investigated the IDOD value change according to the inorganic salts and gained the following results: 1. IDOD value influenced by Na2SO3 is 81.4 ppm. 2. Generally sulfur compounds are highly effecting on IDOD. 3. The nitrite salt had little influence on IDOD.