Oscillator strengths for C 1s and O 1s excitation of gas-phase 1,2-cyclohexanedione, 1,3-cyclohexanedione, 1,4-cyclohexanedione, and cyclohexanone have been derived from electron energy loss spectra recorded under conditions of electric dipole scattering. Spectral assignments have been aided by extended Huckel (EHMO) and HAM/3 calculations. Spectral features characteristic of diketo and monoenol structures are identified. The C 1s and O 1s spectra clearly indicate that 1,2-cyclohexanedione has a monoenol structure in the gas phase while 1,3-cyclohexanedione is predominantly in the diketonic structure in the gas phase. The latter-result resolves an existing literature discrepancy concerning the preferred gas-phase tautomer of 1,3-cyclohexanedione.