The work described in this paper formed part of the European UrbVent project on urban ventilation.Measurements of wind speed, wind direction, and air temperature were made at four different heights, inside a pedestrian street canyon in the centre of Athens, Greece, and at the top of the canyon. In addition, infrared radiation on the canyon faades was measured. Experimental data were collected at intervals of 30 seconds. The dimensions of the canyon were: height/width=2.3, length/height=50/23=2.2 with an orientation of 12 degrees from North. Measurements were made under normal summer weather conditions. Results revealed that there was a significant difference between the surface temperatures of the canyon faades and that the heat island effect varied up to 5C. For ambient flow perpendicular to the main axis of the canyon, the air flow inside the canyon is characterized by a single vortex driven by the ambient air flow. In this case, the air velocity inside the canyon was strongly correlated with the velocity above the buildings. When the air flowoutside the canyon was parallel to the main axis of the canyon, the measured wind speed and direction inside the canyon were strongly correlated with the conditions outside the canyon.