Silicon is known to be an amphoteric impurity in GaAs. A large number of ir absorption bands have been previously reported for Si‐doped GaAs which has been compensated by Li or Cu diffusion. These bands, which are at frequencies above the pure GaAs single‐phonon spectrum, have been attributed to localized vibration modes of defects. The present experimental study extends previous work to show that all the observed bands (frequencies given in parentheses) are explicable in terms of the presence of the following defects: SiGa (384 cm−1), SiAs (399 cm−1) SiGa‐LiGa (374, 379, 405, 470, 480, 487 cm−1), SiGa‐CuGa (374, 376, 399 cm−1), and SiGa‐SiAs. (367, 393, 464 cm−1). These assignments appears to be consistent with results obtained by varying the Si concentration and by different thermal treatments.