The relation between structural perfection and optical properties of InGaN with 10% In are discussed. Transmission Electron Microscopy, X-ray diffraction and Rutherford backscattering spectrometry measurements show that only strained layers with a thickness not exceeding 100 nm are defect free and In concentration is lower than the nominal value. Extension of layer thickness leads to layer sequestration into sublayers with different In contents and the formation of planar defects as a result of layer relaxation. In concentration in such sublayers reach and in some cases exceed the nominal concentration. A single band edge photoluminescence peak is observed only for the thinnest layer. Samples with larger film thickness showed multiple PL peaks corresponding to layers with different In content. Much higher In content would be required to explain the presence of some PL peaks, suggesting that some PL peaks originate from the defective areas of the film. This was confirmed by cathodoluminescence studies performed on the same samples used earlier for TEM studies. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)