Results of experimental investigations on the influence of tube arrangement, velocity and turbulence intensity of the oncoming stream on the temperature distribution in the outlet cross-section of a tube bundle heat exchanger with three rows of plain tubes are presented. The tube bundle heat exchanger was heated by saturated steam condensing inside the tubes and the tubes were cooled by cross flowing air with a homogeneous temperature in the inlet cross-section. In the outlet cross-section the local temperatures were measured by means of a platinum resistance thermometer and a constant-current-anemometer. The measured temperature distributions of the air in the outlet cross-section depend significantly on the air velocity, the inlet turbulence intensity and the tube arrangement. For a pitch-to-diameter ratio of a=2.58 and b=1.29, an inlet turbulence intensity of Tu=0.8% and a mean velocity of uo=10 m/s the temperature distribution of the air in the outlet cross-section could be described as an unmixed flow. Lower pitch-to-diameter ratio e.g. a=1.29, higher mean flow velocities to uo=20 m/s, or inlet turbulence intensities up to Tu=16.6%, leads to temperature distributions as in a partially mixed flow.