The present invention provides a stabilization method, a preservation method and the like method of reduced coenzyme Q.sub.10, which is useful as functional nutritive foods, specific health foods and the like. Furthermore, the present invention provides a method for efficiently obtaining reduced coenzyme Q.sub.10 of high quality and by a method suitable for a commercial production. It is possible to handle and stably preserve reduced coenzyme Q.sub.10 under a condition that oxidation by a molecular oxygen is inhibited by contacting reduced coenzyme Q.sub.10 with, an ascorbic acid and citric acid or a related compound thereof, and thus a stabilized composition is obtained. Moreover, reduced coenzyme Q.sub.10 is converted into a crystalline state in such a condition that the formation of oxidized coenzyme Q.sub.10 as a byproduct is minimized by crystallizing reduced coenzyme Q.sub.10 in the presence of ascorbic acid or a related compound thereof etc., and thus a reduced coenzyme Q.sub.10 crystal of high quality is produced. Furthermore, by successively crystallizing the generated reduced coenzyme Q.sub.10 in the presence of ascorbic acid or related compound thereof after reducing oxidized coenzyme Q.sub.10 to reduced coenzyme Q.sub.10 using ascorbic acid or a related compound thereof, operations are simplified and minimized, and thus reduced coenzyme Q.sub.10 of high quality is produced.

A fluorene-containing resin with a fluorene skeleton and an ester bond and/or a carbonate ester bond is depolymerized by a depolymerization method that includes allowing the fluorene-containing resin to react with a carbonate ester in the presence of a hydrolysis catalyst to decompose the fluorene-containing resin, obtaining a decomposition product. This depolymerization method allows simple or easy depolymerization of the fluorene-containing resin. The decomposition product may contain a monocarbonate ester form of a diol and/or a dicarbonate ester form of a diol. In the depolymerization method, the decomposition product may be allowed to react with an alcohol to obtain a diol.

A fluorene-containing resin with a fluorene skeleton and an ester bond and/or a carbonate ester bond is depolymerized by a depolymerization method that includes allowing the fluorene-containing resin to react with a carbonate ester in the presence of a hydrolysis catalyst to decompose the fluorene-containing resin, obtaining a decomposition product. This depolymerization method allows simple or easy depolymerization of the fluorene-containing resin. The decomposition product may contain a monocarbonate ester form of a diol and/or a dicarbonate ester form of a diol. In the depolymerization method, the decomposition product may be allowed to react with an alcohol to obtain a diol.