We present structural, electrical, and theoretical investigations of self-assembled type-II GaSb/GaAs quantum dots (QDs) grown by molecular beam epitaxy. Using cross-sectional scanning tunneling microscopy (X-STM) the morphology of the QDs is determined. The QDs are of high purity (~100% GaSb content) and have most likely the shape of a truncated pyramid. The average heights of the QDs are 4–6 nm with average base lengths between 9 and 14 nm. Samples with a QD layer embedded into a pn-diode structure are studied with deep-level transient spectroscopy (DLTS), yielding a hole localization energy in the QDs of 609 meV. Based on the X-STM results the electronic structure of the QDs is calculated using 8-band k·p theory. The theoretical localization energies are found to be in good agreement with the DLTS results. Our results also allow us to estimate how variations in size and shape of the dots influence the hole localization energy.