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.