The quaternary GaInNAs alloy is a very promising material system for lasers in the 1.2-1.6 mum range with application in telecommunication fiber-optic networks. While good quality laser material has been demonstrated at 1.3 mum, pushing the emission beyond 1.5 mum by adding up to 40% In and 2% N has been unsuccessful. Recently, the addition of small amounts of Sb has put this alloy back on track for the 1.5 mum challenge by dramatically improving the luminescence efficiency of the material. In this work, high-resolution transmission electron microscopy (TEM), energy-filtered TEM, dark-field TEM, and energy-dispersive x-ray spectroscopy were used to structurally characterize both GaInNAs and GaInNAsSb quantum well structures. The results provide insight into the role of antimony in improving the optical properties of the material, namely reducing the local compositional fluctuations of In. (C) 2004 American Vacuum Society.