InGaAsN/GaAs heterostructures were grown by low pressure MOVPE. Their structural and optical properties were investigated. Pseudomorphic InGaAsN/GaAs layers with a band gap as low as 0.9 eV as well as ternary InAsN quantum dot and well structures were fabricated. It was shown that the addition of a few percent of nitrogen during the formation of the InAs quantum dots led to a decrease in their dimensions and density which was also accompanied by a sharpening of their uniformity distribution. An increase of the nitrogen content reduced the lattice mismatch between InAsN and GaAs. This also gave rise to a layer-by-layer growth mode and smooth heterointerfaces. The InAsN/GaAs multiple quantum well heterostructures so fabricated had improved structural quality and demonstrated intense photoluminescence at room temperature. (C) 2002 Published by Elsevier Science B.V.