Values of absorption constant covering the range 1 cm−1to 104cm−1have been derived from transmission measurements made on single‐crystal gallium arsenide. The absorption edge is very steep up to ∼4000 cm−1, where there is a knee beyond which the absorption increases relatively slowly with photon energy. The energy bands have been calculated using Kane's theory. From these a theoretical absorption curve has been obtained which shows very good agreement with the experimental data.Using semi‐insulating material, it has been possible to measure the shift of the edge with applied electric field. The effect is small (∼200‐&mgr; ev shift for 5000‐v/cm field) but is in good agreement with theory.