In this work undoped semi-insulating (SI) GaAs grown by vertical gradient freeze and liquid encapsulated Czochralski methods was studied by near-infrared absorption (NIRA), thermally stimulated current (TSC) and positron annihilation techniques. The positron experiments reveal both gallium and arsenic vacancies, as well as gallium and arsenic antisites, in the samples. By comparing the results from the TSC and positron measurements, the following relations are found in the defect concentrations: trapT2correlates with the arsenic antisite and trapT5with the arsenic vacancy. The ionized fraction of the arsenic-antisite-relatedEL2defect is obtained from NIRA measurements. The positive charge of these ionizedEL2defects correlates with the net negative charge,3[VGa3−]+2[GaAs2−]−[VAs+],related to the gallium vacancies and antisites and arsenic vacancies detected in positron measurements. The intrinsic defects may thus contribute significantly to the electrical compensation in SI GaAs. ©1997 American Institute of Physics.