The question regarding the role of silicon as a residual impurity in high‐purity GaAs is resolved by the use of careful doping experiments and high‐resolution far‐infrared spectroscopy. The results indicate that silicon is readily incorporated into the material as a simple substitutional donor with a binding energy of 5.854 meV and show that silicon is a major, but not necessarily the dominant, residual donor in high‐purity vapor and liquid epitaxial GaAs.