Infrared absorption at oblique incidence and Raman scattering were used to measure plasmon spectra in highly dopedn‐type GaAs layers grown by molecular beam epitaxy. Plasmon frequency was lower in infrared absorption spectra compared to Raman spectra of the same samples due to the wave vector dependence of the plasmon frequency. The linewidth of plasmon lines was quite similar in infrared absorption and Raman scattering spectra, indicating the small role of Landau damping for the studied range of wave vectors. Infrared plasmon absorption was strongly reduced for films thicker than the wavelength of plasmons, and the wave vector dependence was reduced for very thin films when the electron gas started to acquire quasi‐two‐dimensional character.