Alkali decomposition reaction of PEN was studied in ethylene glycol (EG), alcohols and mixed media of EG and ethereal solvents such as diethylene glycol (DEG), triethylene glycol (TEG) and ethylene glycol monobutyl ether (EGMBE). alkali decomposition reaction of PEN in EG medium proceeds via two stage, a fast initial stage and a slow second stage. And there are some induction periods in the initial stage. The alkali decomposition of PEN in alcohol medium is not effective because treatment temperatures higher than the boiling point of the alcohol cannot be used. KOH is more effective than NaOH in alkali decomposition PEN, but the difference is not so much. The ethereal solvents accelerate the rate of decomposition in the order. EG<DEG<TEG≪EGMBE. With increasing mixing ratio of DEG or TEG, the reaction rate increases, but in case of EGMBE, there is a maximum rate in the vicinity of 50 vol%. The reaction medium containing EGMBE shows different decomposition behavior from DEG or TEG mixed medium, and yields high decomposition rate with significant swelling of PEN surface.