Mechanical properties of polyethylene terephthalate oriented in a series of n-alkanols and containing 30–60 wt.% of the low molecular weight components have been investigated. It was found that the yield point for PETP oriented in the liquid media is a function of the amount of alcohol in the polymer. It is shown that near T g unoriented PETP swells to only an insignificant extent in the same active liquids, which is reflected in a marked reduction in its glass transition temperature. It is thought that two types of plasticization take place in PETP containing n-alkanols: a molecular type caused by a small amount of alcohol linked to polymer, and a structural type stemming from the surfactant action of a large amount of alcohol trapped by the porous structure of the polymer and forming its own individual phase.