Hydroxycycloalkanones 1 of medium ring size (8 -10) exist in a transannular tautomeric equilibrium with the corresponding oxabicycloalkan-1-ols 2, which represent hemiacetals. Normally, the bicyclic structures 2 predominate in solution although their portion decreases with increasing solvent polarity. A correlation of the Gibbs reaction enthalpies ∆G (1→ → →2) with the solvent parameters E T (30) is presented