The present invention relates to a modular breathing gas separator unit (5; 105; 205; 305) for breathing gases, the unit being connectable to a breathing system (1). The unit comprises a first module comprising a gas separation means (25, 125; 224, 225; 350) adapted to separate ventilator driving gas and patient inhalation and/or exhalation gases, and a second module comprising a carrier means (27; 127; 227; 327) adapted to support and connect the gas separator unit to an interface (101) in the breathing system. The disclosure also relates to the breathing system (1) comprising the unit.