The indirect band gap of GaAs1-xPx has been determined accurately over a large alloy composition range. The results were obtained with a wavelength derivative technique in optical absorption on material grown under complete equilibrium conditions. Zero-phonon as well as momentum-conserving phonon-assisted free-exciton transitions were observed. A distinct curvature in the direct band gap-vs-alloy composition curve is determined. Photoluminescence measurements of the same samples, together with the absorption edge data, give the alloy composition dependences of the energies of donor-acceptor pairs and bound excitons; thus the intrinsic nature of the transitions observed in absorption is established. © 1972 The American Institute of Physics.