Synopsis The influence of increasing temperature from 20 to 200 degrees C was studied at the superficial level by electron microscopy and in depth by differential colorimetry and X-ray defraction. The results have been related to moisture content in order to establish the influence of hair-drying treatment which would respect the integrity of the keratin fibre whilst encouraging the deformation of the hair. A critical temperature of 140 degrees C resulted from the studies. Below this temperature little modification of the hair structure was observed which was reversible and linked to the progressive loss of free water. Above 140 degrees C the structural modifications were profound and irreversible. This was accompanied by a change in the appearance of a folding of the cuticle and the gradual disappearance of the scale of the cuticle. After elimination of bound water, a total degradation of the structure was observed around 200 degrees C. From the point of view of these structure studies, a drying temperature of 60 degrees C would appear optimal since a subsequent moisture pick-up is slower, leaving the hair less sensitive to atmospheric humidity variations.