Oil and water mixtures are separated in a gas flotation separator by passing the mixture downward in a vertically elongated portion, and then upward in a vertically elongated portion. A gas such as air is injected near the lower end of the upward flow leg of the separator. The gas is injected at a sufficient hydrostatic pressure to effect dissolving of a substantial portion of the gas in the liquid. The upflow leg has a chamber for oil flotation. Oil removal as well as water removal from respective layers is provided. The separation of oil and water is enhanced by both upwardly bubbling gas and exsolving gas.