Many PVC materials deteriorate only 5 years after manufacture. The extent, rate and mechanisms of deterioration of model and naturally aged PVC containing di (2-ethylhexyl) phthalate (DEHP), have been examined during thermal ageing in various environments. Weight loss was used to quantify loss of DEHP, attenuated total reflection Fourier transform infrared spectroscopy to quantify concentration of DEHP at surfaces and optical densitometry to examine darkening of samples. The rate and extent of deterioration of plasticized PVC and the migration and loss of DEHP plasticizer were related. DEHP inhibited degradation of the PVC polymer, therefore when it was lost, discolouration, tackiness and embrittlement resulted. Less plasticized materials degraded more rapidly than those more highly plasticized. Degradation was inhibited in both model sheets and naturally aged materials by enclosing them in a non-adsorbent material such as glass, containing non-agitated air or storing them in a freezer.