Solid-solid phase transitions of the CL-20 polymorphs epsilon ->gamma, alpha ->gamma, and beta ->gamma were studied isothermally at ambient pressure by transmission FT-IR spectroscopy and analyzed by multivariate regression. Activation energies as function of the extent of conversion were obtained using the model-free isoconversional method. ne apparent activation energies for all three solid phase transformations were about 210 kJ/mol at 5% conversion, and compared well with the calculated lattice energies. The apparent activation energies increased to a maximum of about 400 kJ/mol at 40%, 80% and 60% conversion for epsilon ->gamma , alpha ->gamma, and beta ->gamma, respectively, and then decreased. The mechanisms of the phase transitions of CL-20 appear to be quite complex. Comparisons are made with the phase transitions of HMX.