The synthesis and cationic polymerization of the following bicyclo orthoesters were examined: 4-ethyl-2,6,7-trioxabicyclo[2.2.2]octane, 1,4-diethyl-2,6,7-trioxabicyclo [2.2.2]octane, 4-ethyl-1-phenyl-2,6,7-trioxabicyclo [2.2.2]octane, 4-ethyl-1(4-methoxyphenyl)-2,6,7-trioxabicyclo [2.2.2]octane, and 4-ethyl-1-(4-nitrophenyl)-2,6,7-trioxabicyclo [2.2.2] octane. All the monomers underwent equilibrium polymerization, which was confirmed by the relationships between the polymerization temperature and monomer conversion. The obtained polymers afforded the original monomers via an acid-catalyst treatment with a low reagent concentration in CH2Cl2 at 20 degreesC. The equilibrium monomer concentration was constant, regardless of the initial reagent concentration, in both polymerization and depolymerization. The bicyclo orthoesters with a bulky and electron-withdrawing substituent showed a larger equilibrium monomer concentration.