A process to be used systematically for establishing contact between a gas and a liquid such as might, in particular, react together.The process comprises a series of sequences each comprising effecting the intensive mixing, e.g., the emulsification, of the gas and liquid, expanding the emulsified mixture, separating the gas and liquid and recompressing the liquid. These sequences are in series. Over-all the liquid and the gas move in opposite directions, but within a given sequence the gas moves in the same direction as the liquid.The invention applies to water treatment by a regenerative gas such as ozone.