Abstract: A treater apparatus for separating mixed oil, water and gas into these components and including an elongated tank having a first end and an opposite, second end. A horizontally extending partition plate extends across the first end portion of the tank below a heater tube. The mixture is introduced to a point below one end of the plate, then reverses flow along the heater and over the partition plate until it contacts a vertically extending, transverse corrugated baffle plate. The longitudinal flow of the mixture is reversed several times as it flows over the heater tube and against the crenulated baffle plate. Water separates from the oil, and gas moves to the top of the tank and is removed by a vertical pipe. In the opposite, second end of the tank, a T-shaped partition plate subassembly further separates oil and water, and directs each into a separate removal chamber defined within the tank.