Previous determinations of Landau level lifetimes in GaAs/AlGaAs heterostructures from saturation cyclotron resonance measurements have been confused by heating effects. We have utilized a ps free electron laser to show that for samples with sheet concentration less than 3×10 11cm-2, true saturation of cyclotron resonance is observable at high magnetic fields, in the presence of polaron nonparabolicity. However, at higher concentrations, the polaron is screened and saturation is no longer possible. At low magnetic fields (i.e., long wavelengths) where the polaron nonparabolicity is negligible, saturation is not possible for any sheet density. It is confirmed that the lifetime of the first excited Landau level has an inverse dependence on carrier density.© 1995 American Institute of Physics.