Sn-doped Al x ;Ga 1 - x ; As were studied by photoluminescence. The near-gap emissions consisted of two types of donor-acceptor pair emissions, D 1 -A and D 2 -A. By analyzing the composition dependent characters, D 1 - was associated with a hydrogenlike shallow donor following: the Γ band mininwn D 2 -A was associated with a donor related to the X; band minimum. An additional structure of D 2 -A emission was distinctly visible in high composition of direct gaps as well as of indirect gaps. This line shape was best fitted by the transition involving phonon. The Huang-Rhys factor S and phonon energy ω were determined from the fitting results. The values Franck-Condon shift S ω, representing the magnitudes of the lattice relaxations, increased with increasing composition, which was explained in terms of the local lattice distortion around the ionized D 2 donor. The phonon energy increased significantly from 32 meV to 44 meV with increasing composition around the Γ and X; band minimum crossover region. A local vibrational mode (LVM) of heavy substitution is known to originate from the bottom of an optical phonon band. For A1GaAs: Sn, two optical phonon bands called GaAs-like and AlAs-like modes induce two LVMs of the D 2 donor. In low compositions the D 2 is mainly surrounded by Ga atoms, yielding high-state densities of the LVM arising from the GaAs-like mode, whereas the high-state densities of the LVM resulting from the AlAs-like mode exist in high compositions. The variation of phonon energy is attributable to a weighted average of the two LVM frequencies.