The cyclotron resonance (CR) of electrons confined in a thin InAs quantum well sandwiched between thin GaSb layers has been studied experimentally. Since the transmission of radiation in thin polar layers is suppressed only near the TO frequency, we can tune the CR through the TO-LO frequency region of both materials. We find that there is a drastic influence of TO phonons on the CR line profile, resulting in split CR. This interaction is shown to be dominantly of optical origin; resonant polaron effects, both at TO or LO frequency, are, if present, very small. © 1987 The American Physical Society.