High temperature Hall effect and resistivity measurements have been made on semi-insulating 4H-SiC samples. Both vanadium doped and undoped materials have been studied. Resistivity measurements before and after annealing up to 1800 °C are also reported. The thermal activation energy of the resistivity in vanadium doped samples has one of two values, 1.5 and 1.1 eV, due, respectively, to the vanadium donor level and an as yet unidentified defect. The activation energies for high purity semi-insulating material (HPSI) varied from 0.9 to 1.5 eV. Hall effect measurements were made on several HPSI and 1.1 eV V-doped samples. In all cases the material was found to be n-type. Mixed conduction analysis of the data suggests that the hole concentration is negligible in all samples studied. This suggests that the defects responsible for the semi-insulating properties have deep levels located in the upper half of the bandgap. The resistivity of V-doped samples were unaffected by anneals up to 1800 °C. The annealing results for HPSI samples were mixed. © 2004 Elsevier Ltd. All rights reserved.