The selection of appropriate characterization methodologies is vital for analyzing and comprehending the sources of defects and their influence on the properties of heteroepitaxially grown III-V layers. In this work, we investigate the structural properties of GaAs layers grown by metal-organic vapour phase epitaxy on Ge substrates - (100) with 6° offset towards 111 - under various growth conditions. Synchrotron x-ray topography is employed to investigate the nature of extended linear defects formed in GaAs epilayers. Other x-ray techniques, such as reciprocal space mapping and triple axis ω-scans of (00l)-reflections (l = 2, 4, 6), are used to quantify the degree of relaxation and presence of antiphase domains (APDs) in the GaAs crystals. The surface roughness is found to be closely related to the size of APDs formed at the GaAs/Ge heterointerface, as confirmed by x-ray diffraction (XRD), as well as atomic force microscopy and transmission electron microscopy. © 2012 IOP Publishing Ltd.