Process for reducing CO2 and membrane separation arrangement. A process/arrangement for reducing the CO2 content of a gas mixture using four membrane separation steps, two of the separation steps using glassy membrane and two using rubbery membrane. A first retentate is withdrawn from the first separation step at a first withdrawal pressure and temperature and is supplied to the second separation step, a second retentate is withdrawn from the second separation step at a second withdrawal pressure and temperature and is supplied to the third separation step, a third retentate is withdrawn from the third separation step at a third withdrawal pressure and temperature and is supplied to the fourth separation step, permeates of the separation steps using the glassy membrane are combined to form a first combined permeate stream, and permeates of the separation steps using the rubbery membrane are combined to form a second combined permeate stream.