Crosslinked poly(cyclic orthoester)s prepared by radical additions of poly(cyclic orthoester)s possessing exomethylene groups and dithiols efficiently depolymerized to bifunctional spiro orthoesters in the presence of trifluoroacetic acid in dichloromethane. Furthermore, the dithiol-linked bifunctional spiro orthoester monomers prepared by the radical additions of spiro orthoester possessing exomethylene group and dithiols afforded the corresponding crosslinked polymers in the presence of trilluoroacetic acid as a catalyst in bulk condition. The quantitative depolymerization of the obtained crosslinked polymer afforded the corresponding bifunctional monomer in dichloromethane at room temperature. Meanwhile, an acid-catalyzed reversible crosslinking-depolymerization of a polymer having a spiro orthoester group in the side chain was carried out. The copolymer obtained by the radical copolymerization of 2-methylene-1,4,6-trioxaspiro[4.6]undecane with acrylonitrile was treated with trifluoroacetic acid in dichloromethane at -10°C to atford the crosslinked polymer quantitatively. The crosslinked polymer was then treated with trifluoroacetic acid at a low concentration in dichloromethane at room temperature to recover the original polymer.