Plasticised poly(vinyl chloride) (PVC) is found in museum collections both as three-dimensional objects and packaging materials. Many PVC materials deterioratate within 25 years of acquisition. Migration and chemical breakdown of plasticizer, accompanied by dehydrochlorination of the polymer, are the major mechanisms of deterioration. The extent, rate and mechanisms of deterioration of new and naturally aged PVC containing di-(2-ethylhexyl)-phthalate (DEPH), the most commonly used plasticizer, were examined during accelerated thermal ageing in various environments. Weight loss was used to quantify loss of plasticiser. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy was used to quantify concentration of DEHP at surfaces. Optical densitometry was used to quantify darkening of samples. Degradation of new and deteriorated PVC was inhibited by enclosure in a non-adsorbent material such as glass, containing un-stirred air. Such storage minimised migration and loss of DEHP, thereby prolonging the useful lifetime of PVC objects tenfold.