Abstract: A heat exchanger (4) for cooling a machine, such as a mechanical gear or some other transmission component, hydraulic pump, motor or clutch, compressor, internal combustion engine or like machine, which comprises a housing (1) in which movable, mechanical elements (2), which are driven when the machine is working, are housed, and which encloses a liquid medium (3), such as a lubricating or hydraulic liquid. The heat exchanger is mounted in the outer wall (5) of the housing and comprises two chambers (6, 7) which are mutually separated by a liquid-impervous partition wall (8) which comprises part of the housing wall. The inwardly located chamber (7) is through-passed by the liquid medium (3) enclosed in the housing, while the outwardly located chamber (6) is through-passed by a cooling medium.