Mixtures of O3 in excess O2 and N2O were photolyzed with 3130-¿ radiation at 293¿, 258¿, and 221¿K. From the quantum yields of N2 production the quantum efficiencies for O(1D) production were found to be 0.29, 0.22, and 0.11, respectively, at the three temperatures, with an uncertainty of 0.04 in each result. The temperature dependence was attributed to the vibrational and rotational energy needed in the O3 molecule to surpass the energy requirement for O(1D)+O2(1Δ) production. Since angular momentum must be conserved during dissociation, only two degrees of rotational freedom are available, and the data are best-fitted with a critical rotational-vibrational energy of 0.86 kcal/mol. This leads to a dissociation enthalpy of 24.32¿0.1 kcal/mol for O3 at O¿K.