The separation of crude oil and water mixtures is an important process in the oil and chemical industries. This work studied the flow behavior of crude oil and water in a pilot scale oil-water separator. This gravity separator (diameter of 1.2 m and length of 5.2 m) was operated by Drood oil of the Iranian Offshore Oil Company (IOOC) located in Kharg Island (Iran). The residence time distribution (RTD) curves were acquired in this separator by radioactive tracer (1311) injection. Experimental results showed that the separator operational performance increased with the water level in the vessel. Perfect mixing tanks-in-series (with a dead zone) have been used to describe the liquid behavior, and the experimental results were in good agreement with this model. Increasing the water level in the vessel from 0.5 m to 0.9 m increases the number of mixing tanks-in-series of both organic and aqueous phases, from 9.0 to 9.1 and from 8 to 8.3, respectively. The dead volume of the organic phase path decreased with water level, but this trend was reversed for the aqueous phase path. Less than 3% of the volume of the separator is active. The separator's optimum efficiency occurred when the water level in the separator was about 0.7 m. (C) 2008 Elsevier B.V. All rights reserved.