We report the dynamic mechanical behaviour of leather within the temperature range-100 to 300 C, where three major viscoelastic transitions were identified, termed α, β and γ. The β transition peak represents the glass transition temperature of the amorphous region of collagen molecules. It also demonstrates that tanning agents act as plasticisers and depress the glass transition temperature to a lower temperature. Thus, the tanning process itself may be viewed as a plasticisation of the collagen molecule: the tanning molecules interpose themselves between the collagen chains, thus reducing the forces holding the chains together. Different tanning agents show differing degrees of plasticisation.