An apparatus for boosting the pressure of flowing fluids includes jet pumps (10a, 10b), each having a low pressure (LP) inlet for LP fluid, a high pressure (HP) inlet for HP liquid and a medium pressure (MP) outlet for MP fluid. A fluid separator device (16) receives the MP fluid from the outlets of the jet pumps, and includes a gas outlet line (20) for a separated gas phase and a liquid outlet (22a, 22b) for separated liquid phase. A liquid return line (24b, 38) returns at least some of the separated liquid phase to the HP inlets of the jet pumps, and a mechanical pump (32) is connected into the liquid return line for boosting the pressure of the liquid delivered to the HP inlets of the jet pumps. A flow control system (6) is provided for controlling the flow of fluids through the respective jet pumps (10a, 10b).