GaAs layers were deposited onto Si(100) substrates using molecular beam epitaxy (MBE). The substrates were prepared by various methods before the growth of 0.1µm thick GaAs buffer layers. Active GaAs layers were thereafter grown with thicknesses from 1 to 4µm. Infrared spectra were recorded in the spectral ranges from 12 to 65cm-4 and 50 to 500cm-1 of the thin buffer layers as well as the thick active layers. The infrared active phonon of GaAs already shows up in the spectra for the 0.1µm buffer layers. The phonon parameters were derived from a harmonic oscillator fit to the experimental data. The phonon damping constant is correlated with the bulk quality of the GaAs layers. These results are compared with those obtained from Raman and spectroscopic ellipsometry measurements.