This study investigates the mechanical behaviour of polyester fibres over a range of temperatures by the use of a number of experimental techniques including tensile, creep and fatigue tests. The influence of temperature on the different mechanical properties is investigated with particular emphasise on the loss of rigidity and reduction of creep and fatigue lifetimes. A precise description of the stress/strain curves of the fibres highlights the effects with respect to the glass transition temperature. The fatigue failure process is precisely detailed and changes, at high temperature, to the classical fracture morphology observed are presented. A comparison between observations made of single fibres and yarns provides valuable information about the interpretation of results obtained with single fibre test to fibre assemblies.