A reactor produces a gas-in-liquid emulsion for providing increased interfacial contact area between the liquid and the gas for improved reaction of the gas with the liquid, or more rapid solution or reaction of a difficulty soluble or immiscible gas in or with a liquid. The reactor is suitable for a continuous or batch type process. Rotor and stator cylindrical members are mounted for rotation relative to one another and have opposing surfaces spaced to form an annular processing passage. The gap distance between the opposing surfaces and the relative rotation rate of the cylindrical members are such as to form a gas-in-liquid emulsion of the gas in the liquid. The liquid and gas pass through the processing passage, changing into the gas-in-liquid emulsion.