The invention relates to a heat exchanger, especially a gas cooler for CO2, embodied as a cooling agent. The heat exchanger comprises at least one two-part collector unit made of a base and a cover. Said collector unit consists of flat pipes and at least two longitudinal channels with an essentially circular cross-section. The ends of the flat pipes and the base comprise openings for receiving the ends of the pipes. The base, cover and flat pipes are soldered together.