High-level ab initio calculations of the structure, vibrational frequencies, and NMR spectra of the recently isolated methyl hypofluorite, CH3OF, have been carried out. When electron correlation is included in the calculations (but not at the HF level), there is a very good agreement between the experimental and the theoretical IR and NMR spectra. Four different unimolecular decomposition pathways, all leading to CH2O and HF were studied. Of these, two mechanisms, the synchronous single-step HF elimination and a two-step mechanism via the CH3O. and F-. radicals, are predicted to be the most favorable, both having activation free energies of ca. 38 kcal mol(-1) at GAUSSIAN 2. A theoretical analysis of the expected kinetic isotope effects between the competing pathways leads to a clear differentiation which can be used in experimental studies.