Reflection and transmission spectra were recorded for various temperature for two different polarizations from 5 to 600 meV. We studied the plasmon-phonon coupling in ZnO : Ga single crystals, for foe carrier concentrations of up to 1.8 x 10(20) cm(-3) Our data show that the position of the plasma edge in the reflectivity spectra is highly sensitive to the method of preparation of the ZnO surfaces.