Temperature and velocity profiles were measured in a flat-plate boundary layer subjected to free stream turbulence intensities ranging from 1 to 50% and at Reynolds numbers from 8 400 to 127 000. The greatest turbulence intensity values were accompanied by large scales instabilities into the flow. When Tu∞≈1% the temperature and velocity profiles were in agreement with the Blasius solution providing that the real boundary layer thickness was taken into account. For Tu∞⩾6% the thermal and velocity boundary layers greatly thickened as well as the displacement and momentum thicknesses. A “wake region” was observed in the profiles and there was an increase of slopes in the near-wall region of the thermal profiles. Moreover turbulent energy was produced in the boundary layer. Some differences were observed between the thermal and velocity boundary layers which can be explained by the unheated starting length which existed at the leading edge of the plate.