A detailed photoluminescence (PL) study reveals that the low-temperature PL emission in GaNAs epilayers and GaAs/GaNxAs1 - x quantum well structures grown by molecular beam epitaxy is governed by recombination of localized excitons. This conclusion is based on the analysis of the PL lineshape, its dependence on the excitation power and measurement temperature, as well as PL transient data. The depth of the localization potential is estimated as about 60 meV, varying slightly among the different structures.