A piece of poly(tetrafluoroethylene) (PTFE) enclosing NaCI powder and having the dimensions shown in the inset in Fig. 1 was placed along with water in a deformable gold cell in a Dickson-type hydrothermal apparatus, and heated stepwise up to 410℃ under a constant pressure of 1 kbar. During the heating, small fractions of the solution in the cell were sampled and analyzed gas chromatographically for H(2), O(2), CH(4) and CO(4), and ion chromatographically for F(-) and Cl(-). No evolution of gases (H(2), CH(4), CO(2)) due to decomposition of PTFE, other than that due to decomposition of organic impurities, was observed over the temperature range of the experiment. The ion chromatographic analysis showed (Fig. 1) that : (1) Cl(-)leaching from the PTFE test piece continued even at 340℃ ; (2) F(-) leaching was small in amount and completed within the first step at 200℃ and 12 hours ; (3) F(-) formation due to partial decomposition of PTFE became measurable from 340℃, was linear with time, and was remarkably accelerated at temperatures above 400℃ ; (4) migration of the enclosed NaCl (partly hydrolyzed during the course of the experiment) did not occur even at 410℃ ; but (5) osmosis of water caused a puncture of the test piece within 2 hours after the temperature reached 410℃. The test piece recovered after the run was found to be retaining the original luster and hardness.