Bright-field and high-resolution transmission electron microscopy and microdiffraction have been used for the study of defects in two HgTe/HgCdTe single quantum well (QW) structures grown by molecular beam epitaxy on GaAs substrates with ZnTe and CdTe buffer layers. Defects in QW layers were identified as stacking faults, dislocations, dislocation loops and lattice deformations. The importance of an extra HgCdTe layer placed between the CdTe buffer and HgTe/HgCdTe QW structure for the reduction of defect density both in the barrier layers and in the well itself was demonstrated.