Phonon‐assisted tunneling (PAT) has been studied in detail for two similar GaAs/AlAs double‐barrier structures. Calculations of the PAT current—including effects of optical‐phonon localization—are in good agreement with experimental data, and the emission rate for certain phonon types is found to depend sensitively on GaAs well width. We find that GaAs‐like modes clearly dominate in structures with wider wells, while GaAs and AlAs‐like modes contribute equivalently in narrower well structures. A simple overlap integral—involving the phonon potential and electronic wave functions—provides an effective selection rule for determining which types of phonons are preferentially emitted. ©1995 American Institute of Physics.