Polyfluoroethylenepropylene (PFEP) film was pyrolysed both in air and nitrogen stream at relatively low temperature range from 350°C to 430°C. It was found by in-frared analysis that major components of the gaseous pyrolysis products were carbonyl fluoride, trifluoroacetyl fluoride, tetrafluoroethylene and hexafluoropropylene in air stream and tetrafluoroethylene and hexafluoropropylene in nitrogen stream. Hydro-lysable fluorides such as carbonyl fluoride and trifluoroacetyl fluoride were quantita-tively determined as fluorine ion content by alizarin complexone colorimetry. A profile that amount of the hydrolysable fluoride increased with pyrolysing time at each tem-perature was in fairly good agreement with the profile that amount of weight loss of PFEP increased with pyrolysing time. Amount of the weight loss of PFEP by pyrolysis was greater in air stream than in nitrogen stream. About 65% of the fluorine of PFEP was converted to the hydrolysable fluoride on the pyrolysis in air stream at the above-mentioned temperature range. Among three kinds of boat material tested, the stainless steel boat was found usable without significant loss of the hydrolysable fluoride in comparison with the repeatedly used monel boat. Amount of hydrolysable fluoride remaining on the inner wall of tube was not significantly great as compared with that produced.