Poly(vinyl chloride) (PVC) samples prepared at various temperatures have been submitted to a substitution reaction for purposes of selectively removing the GTTG-TT conformations of the mmr termini of long isotactic sequences. The physical ageing of all samples has been studied by means of differential scanning calorimetry techniques. Surprisingly, a straight relation between the loss of enthalpy and the above GTTG-TT content, which pass through a minimum, has been found. Such a novel relationship should further lead to better understanding of the reasons why polymeric materials experience nonequilibrium states which are known to cause physical ageing.