In a recent publication the authors reported investigations on the relaxation behaviour of isotactic polypropylene fibres in the alpha-transition range. The results of the analysis of relaxation curves on the basis of a two-component model were incompatible with those of a previous investigation by Faucher, who found thermorheological simplicity (TRS) for the material. This paper investigates this incompatibility, by checking whether the relaxation curves previously obtained can alternatively be systemized on the basis of the TRS principle. It is shown that the curves superimpose to yield a satisfactory master curve. However, calculating dynamic mechanical properties, i.e. tan delta, from this master curve leads to a pronounced inconsistency with experimental results. Thus it may be concluded that the TRS principle is only an empirically successful data reduction method that does not adequately model the time-temperature superposition for semicrystalline polypropylene and is hence not applicable for a reliable prediction of its long-term viscoelastic performance.