Structural defects in InSb/Al(x)ln(1-x)Sb quantum-well samples grown on (0 0 1) GaAs substrates have been investigated using transmission electron microscopy. A high density of dislocations is created at the interface between the InSb nucleation layer and the substrate, while {1 1 1}-planar defects are introduced at various locations. An InSb/AlxIn1-xSb interface 920 nm above the InSb/GaAs interface acts as a trapping site for many dislocations. The majority of the {1 1 1}-planar defects are micro-twins with {1 1 1}-bilayer thickness fluctuations. The anisotropic distribution of the micro-twins correlates with the measured anisotropic electron mobility in the quantum well. (C) 2002 Elsevier Science B.V. All rights reserved.