We have studied the longitudinal-optical (LO)-phonon–plasmon coupled modes in high-mobility InN layers with free-electron densities ranging from to by means of Raman scattering. The observed coupled-mode peak displays the usual behavior for the low energy branch of the long-wavelength coupled modes, increasing in frequency and phononlike character as the electron density increases. The mode behavior can be satisfactorily explained by the standard dielectric model developed by Hon and Faust which takes into account wave-vector conserving scattering processes governed by the dipole-allowed deformation potential and electro-optic mechanisms. The free-electron density obtained from line-shape fits to the peak agrees well with Hall-effect measurements. The mode shifts to lower frequencies as the electron density increases, suggesting that strain relaxation has a bearing on the residual electron density in the InN layers. The frequency exhibits also a dependence on the excitation wave vector, which further indicates that wave-vector conserving scattering by LO-phonon–plasmon coupled modes takes place in these high-mobility samples. The presence of a relatively strong LO signal is attributed to surface-field-induced scattering in the accumulation region.