GaAs layers grown by molecular‐beam epitaxy (MBE) at very low substrate temperatures have gained considerable interest as buffer layers for GaAs metal–semiconductor field effect transistors (MESFET’s) due to high resistivity and excellent device isolation. However, the structure and the electronic properties of such layers have not yet been investigated in detail. We have studied unannealed low temperature (LT) MBE layers grown at 200 °C using transmission electron microscopy (TEM), analytical TEM, x‐ray diffraction, the Hall effect, and electron paramagnetic resonance (EPR) techniques. TEM data indicated large arsenic‐rich deviations from stoichiometry of ∼1–1.5 at. %. X‐ray rocking curves showed a uniform increase of 0.1% in all directions of lattice parameters compared to semi‐insulating GaAs substrate. The Hall effect and thermally induced changes of photo‐EPR measurements revealed the presence of an acceptor level at an energy of ∼0.3 eV above the valence band. This acceptor level has been tentatively attributed to a gallium vacancy defect.