The failure of unnotched PVC and PVC-CPE was studied in a number of vapour and liquid environments. The failure mechanisms observed are related to craze initiation and logarithmic craze growth. A model to explain the origin of the logarithmic craze growth is presented. The failure mechanism in air and in benzene and toluene vapours is ductile, whereas it is brittle in n-hexane, n-decane and ethanol vapours, n-octane/benzene mixtures and natural gas condensate. Equations to describe the time to failure for both failure mechanisms are derived. The ductile failure is ascribed to plasticization and/or plastic deformation around the craze. Brittle failure is thought to arise from crazes which reach a critical size.