Two types of conformational equilibria and the Delta H for their interconversion have been investigated by 2 + 1 resonance enhanced multiphoton ionization (REMPI) of jet cooled 4-methyl and 4-ethyl cyclohexanone. The axial-equatorial interconversion equilibria have been investigated for both molecules, while the additional interconversion of the ethyl rotor conformations have been studied for ethylcyclohexanone. van't Hoff plots (ln K-eq vs 1/T) obtained by varying the pulsed valve temperature have yielded the Delta H-ax/eq and Delta H-rotor for both equilibria. The measured enthalpy differences for axial/equatorial conformers were 2.1+/-0.20 kcal/mol for 4-methyl cyclohexanone, and 2.2+/-0.20 kcal/mol for 4-ethyl cyclohexanone. The Delta H for the ethyl rotor in 4-ethyl cyclohexanone was measured to be 1.1+/-0.10 kcal/mol. In the case of the axial-equatorial equilibrium, the van't Hoff plot was linear up to the highest valve temperatures investigated (200 degrees C). This demonstrates that in the supersonic expansion the vibrational cooling is much more rapid than axial-equatorial interconversion. On the other hand, the van't Hoff plot for the ethyl rotor equilibrium leveled off at 80 degrees C. This demonstrates that above this temperature, the ethyl rotor equilibration rate is faster than the vibrational cooling rate. In all cases, the final vibrational temperature, as measured from the absence of spectral hot bands is below 50 K.