The defect structure of molecular beam epitaxy grown InGaAs/GaAs(001) heteroepitaxial systems is studied by using selective electrochemical etching. The InAs mole fraction varied between 0.03 and 0.3 whilst the layer thickness changed between 0.02 and 0.5 mu m (above the critical layer thickness). Different aqueous electrolytes and bias voltages were applied to achieve optimum etching conditions. By incremental layer removal. the depth profile of the dislocation density was mapped. The density of defects is inversely proportional to the layer thickness and increases with misfit. The experimental results are compared to theoretical models.