Human hair is a highly complex biomaterial. For the analysis of its mechanical and thermal properties it is, however, well described by a two-phase structure, which contains as morphological components the highly-ordered, crystalline intermediate filaments (IFs) and the less-ordered, amorphous matrix. A previous study has shown that wool compared to human hair exhibits for the whole range of water contents higher values for the glass transition temperature, T-g, as a property of the matrix. This is despite a higher content of hydrophobic, high glycine-tyrosine (HGT) proteins contained within the matrix of human hair. On this basis, the internal plasticization (IP) hypothesis was developed, stating that the composition of the matrix is the controlling factor of the glass transition and that, namely, the HGT-proteins may have a plasticization effect in the matrix. To further evaluate this hypothesis, esterifications of the hair proteins with various alcohols were carried out to introduce additional and variable hydrophobic sites into the fiber. The effects of these modifications on the humidity-dependent Tg of hair are analysed on the basis of the Fox-equation. It is shown that all esterifications lead to a systematic decrease of, the T-g-values in line with the IP-hypothesis. (C) 2015 Elsevier B.V. All rights reserved.