The low‐temperature photoluminescence of undoped GaxIn1−xAs layers, lattice matched to InP, grown by liquid‐phase epitaxy and molecular‐beam epitaxy has been studied. In this work, we focus mainly on the origin of the line broadening of the two main emissions (excitonic and donor‐acceptor pair transitions) observed. We find that, as it has been recently shown in GaxAl1−xAs, the dominant broadening mechanism is alloy broadening, due to random cation distribution. This model gives linewidths of the bound exciton and the donor‐acceptor pair transitions based on compositional fluctuations within the crystal volumes which are characteristic of the two transitions. Calculated linewidths agree rather well with experimental results, thus demonstrating that alloy broadening leads also in GaxIn1−xAs to a quantitative understanding of the low‐temperature photoluminescence spectra. Careful analysis of the donor‐acceptor pair band yields acceptor activation energies of 15, 22, and 25 meV which are attributed to C, Zn, and Si, respectively.