Abstract: In separation of a stream of oil, gas and water into the constituents thereof in a horizontal heater-treater vessel having a gas separation chamber and an emulsion treating chamber, the stream is caused to flow downward into the gas separation chamber and through an outer secondary heating zone therein to permit gas to separate for upward flow and to cause heated components of the stream to turn to upward flow and thereafter to flow downward through an inner primary heating zone. Common boundaries of the heating zone are defined by an inverted U-shape shroud in the gas separation chamber. Water flows down to the bottom of the vessel from the stream in both the primary and secondary heating zone. Gas flows upward from both the primary and secondary heating zones and an oil-water emulsion flows from the primary heating zone directly into the emulsion treating chamber.