Human hair, like other α-keratinous fibres, 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. The presence of elastic IFs alongside the viscous matrix, results in the viscoelastic properties of human hair. Investigated here are the viscoelastic properties of human hair after two academically and commercially relevant chemical modifications: esterification as well as disulfide reduction and alkylation. Hair fibre properties analysed include moisture sorption, glass transition and thermal denaturation as well as mechanical and dynamic mechanical properties. Attempts have been made to determine the degree of modification using Fourier transform infrared (FTIR) spectroscopy and high performance liquid chromatography (HPLC).Esterification produces a considerable plasticisation effect in the matrix, supporting the internal plasticisation hypothesis put forward from a previous investigation, and based on the comparison of the glass transitions of human hair and wool. Based on thermal denaturation data, esterification is also shown to affect the stability and integrity of the IFs. The effects of alkyl chain length of the ester introduced are related to both plasticising efficiency of the alkyl chain and degree of modification. Disulfide reduction and alkylation are shown to have a severe effect on the matrix, where both disulfide bond cleavage and plasticisation by the introduced alkyl chains contribute to the observed decrease in the water content-dependent glass transition. Sizeable changes in the IF stability and integrity are shown by substantial reductions in denaturation enthalpies. However, it appears that at high water contents hydrophobic interactions between the alkyl chains are sufficient to kinetically impede the unfolding of the IFs to a similar extent as an untreated matrix. This is shown by a largely unchanged denaturation temperature after modification. Some restoration of stability within the fibre has been observed in mechanical tests after n- and i-propylation. Dynamic mechanical properties agree well with data relating to other fibre properties after esterification as well as disulfide reduction and alkylation. Dynamic mechanical properties and their change with temperature, further suggest a role of hydrophobic interactions in stabilising the hair fibre at higher RHs and water contents. A further separation of the contribution of both the matrix and IFs to fibre properties and the effects of the modification on each will in future investigations hopefully provide a deeper understanding of the specific effects of the modifications and also of the interactions important for hair fibre stability.