Unsteady phenomena were observed in the case of airflows approaching transition inside a uniformly heated vertical tube. An analysis of the experimental data at three Reynolds numbers (Re = 1000, 1300 and 1600) allowed to establish that: (i) A flow instability onsets at different Grashof numbers (Gr) when the condition Gr/Re > 1500 is achieved. (ii) The FFT of the temperature signal shows a peak at a frequency f(D) (0.45 Hz) when Gr/Re > 1500. (iii) This instability is associated to a thermal buoyant instability rather than to a thermal shear instability. (iv) This instability forms in the buffer region and then propagates towards the whole section of the tube as the Grashof number is increased. (c) 2008 Elsevier Ltd. All rights reserved.