AbstractAcid‐catalyzed cyclic ketal formationvs.hydrolysis of a series of hydroxyalkyl cyclic enol ethers in the presence of 1 equiv. of H2O, and acid‐catalyzed cyclic‐ketal formation (same ketals as above)vs.methanolysis of a series of mixed pent‐4‐enyl hydroxyalkyl ketals withN‐bromosuccinimide in the presence of 4 equiv. of MeOH led to the same result: the intramolecular cyclization processes occur at similar rates as the intermolecular H2O or MeOH attacks independently of the size of the rings formed (five‐, six‐, or seven‐membered), by cyclizations. These results can be explained by the facts that, due to stereoelectronic effects which impose a torsional strain to the sp2hybridized O‐atom, the cyclization activation enthalpy decreases, as the length of the hydroxyalkyl chain increase (ease of cyclization: 7>6>5), whereas the entropy factor favors the cyclization in the reverse fashion (ease of cyclization: 5>6>7). The various reaction pathways have been examined using the semi‐empiricalHamiltonianAM1, and the results obtained confirm that large‐ring formation is enthalpically much favored over the cyclization processes leading to small rings (ease