Reflectance measurements from p-type GaSb: Zn epitaxial films with different hole concentrations(1017-1018 cm−3) have been investigated over the frequency region of 100-1000 cm−1. A minimum broadening feature corresponding to the hole plasmon was observed in the reflectance spectra. The experimental infrared spectra were well fitted using a Lorentz-Drude dispersion model. The real part ε1 of the dielectric function decreases with increasing hole concentration. However, the imaginary part ε2 increases with hole concentration in the far-infrared region. This indicates that the acoustic- and optic-phonons mainly participate in the free carrier absorption processes. The hole mobility obtained from Hall-effect measurements is slightly larger than that derived from optical measurements and the average ratio of mobilities is estimated to be 1.33. Owing to overdamping effects, the upper branch of longitudinal-optical phonon plasmon (LPP) coupled modes was observed. The upper LPP+ frequency increases with hole concentration and it shows a transition from phonon-like to plasmon-like behavior. A theoretical analysis with solutions in the complex frequency plane describes these experimental results.