The incorporation of silicon into VGF-grown GaAs is examined by Hall effect measurements, spark source mass spectrometry and photoluminescence (PL). It is found that the silicon is incorporated into the crystal according to Scheils-law with the Si concentration [Si] rising from 1.5 × 1018 to 1 × 1019cm-3. It is found that the intensity of the PL peak with energy close to the band gap decreases with increasing Si content of the material, whereas the intensity of the PL peak related to the acceptor SiGaVGa shows opposite behaviour. A compensation model which takes into account the acceptors SiAs and SiGaVGa is developed. The model describes the relationship between [Si] and the charge carrier concentration n up to silicon concentrations of 1 × 1019cm-3 in GaAs grown under low thermal gradients. © 2002 Elsevier Science B.V. All rights reserved.