When (100) wafers of undoped semi‐insulating LEC are etched in the AB etch, both grooved etch features and ridges are produced at dislocations. Both these features have been studied by a correlation of etching, scanning electron microscopy in the wavelength dispersive cathodoluminescence mode (SEM‐WDCL), and transmission electron micros‐copy (TEM). It has been shown that a groove corresponds to a dislocation surrounded by matrix with a relatively slower etch rate whereas a ridge corresponds to a dislocation surrounded by matrix with a higher etch rate. These areas of different etch rate are shown by SEM‐WDCL to correspond to areas with slighly different Fermi levels . The AB etch rate dependence on is confirmed by determining the etch rate for samples with widely separated Fermi levels. An alternative explanation which ascribes grooved features to heavy decoration with As precipitates is shown to be inconsistent with TEM observations.