In order to investigate the extent of error caused by synthetic detergents in oil determination of industrial drainings and to take measurement with accuracy, An author performed this experiment and considered the recovery rate of each tested oil and a recovery variation according to kinds and concentrations of added synthetic detergents and also a recovery change caused by destruction of emulsion. 1. Employing 10 oils and 5 synthetic detergents as testing materials, the changes of n-hexane extracting weight according to kinds and concentrations of added synthetic detergents were examined. 2. The recovery rates of heavy oils such as Bunker-A, Bunker-B, Bunker-C, Rpm #300 and vegetable oil namely soy bean oil were relatively high i. e. 80 to 97%, and the recovery rates of light crude oils such as Indonesia Crude, Kuwait erude, Iranian light were low 25 to 50%. 3. The changes of n-hexane extracting weight were slight according to kinds of each synthetic detergents but were reduced according to the concentration of synthetic detergents. 4. Both of salting out and heating process were performed to destroy emulsion and then the recovery rates of each oil were greatly grown.