The electrical and infrared optical properties of reactively d.c. sputtered polycrystalline ZnO:Al films grown at varying oxygen flow rates were studied on samples prepared in the same deposition run. The carrier concentrations determined by Hall measurements were in the degeneration range. Relating them with the optical free-carrier resonance frequencies the effective electron mass was higher than in non-degenerate ZnO. The values of the optical mobility, calculated from the Drude damping factor, and of the Hall mobility were in the same order of magnitude. This suggests that the low carrier mobility in these films is mainly caused by scattering processes taking place within the film grains and that grain boundary scattering plays a minor role.