The acidity and aggressive properties of volatile distillates in the synthesis of polyethylene terephthalate from ethylene glycol and dimethyl terephthalate have been studied on industrial technological lines. It has been shown that the acidity of the distillates increases as the synthesis process approaches completion. The composition of some of the acidic impurities in the distillates has been identified. It has been shown that the aggressive properties of distillates are determined by their content of strong acids. The effect of the acidity of the original ethylene glycol on the aggressive properties of distillates has been described. It has been shown that the use of neutral ethylene glycol sharply reduces the aggressive properties of the distillates. Measures have been recommended for reducing the aggressive action of distillates and of their distillation products on apparatus and pipelines of nonalloyed steels or cast iron.