AbstractTreatment of 2,7,8‐trioxabicyclo [3.2.1] octane (I) with electrophilic initiators gave polyorthoester composed of five‐ (70‐86%), six‐, and seven‐ (the sum, 14‐30%) membered rings. Treatment of 2,8,9‐trioxabicyclo [3.3.1]nonane (VI) with electrophilic initiators gave mainly oligomers. Trifluoromethane sulfonic acid in dichloromethane was particularly effective for the formation of dimer, 2,8,10,16,17,18‐hexaoxatricyclo[11.3.1.1]hexadecane. The analysis of the structures of poly‐I and oligomer‐VI suggests that I and VI are polymerized thro