Measurements of absorption constant covering the range 1 cm −1 to 10 4 cm −1 have been 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 we have obtained a theoretical absorption curve which shows very good agreement with the experimental data.