The energy difference between the open and the ring isomer of ozone as well as the dissociation energy ( O 3( X ̃ , 1 A 1) → O 2( X, 3 Σ − g )+ O( 3 P)) have been determined at the CCSD(T), MR-CISD and MR-AQCC levels of theory. Using correlation consistent basis sets up to quintuple-zeta quality, the estimated complete basis set limits for CCSD(T) and MR-AQCC lie within 1 kcal/mol of the experimental value of 26.1±0.4 kcal/mol and place the ring isomer by 4.8 and 5.3 kcal/mol, respectively, above the dissociation limit. Zero-point vibrational corrections increase the latter two values by 1.4 kcal/mol.