Electron paramagnetic resonance (EPR) was used to study defects in semi-insulating (SI) SiC substrates grown by high-temperature chemical vapour deposition (HTCVD) and physical vapour transport (PVT). The C vacancy, Si antisite and several other EPR centers, labelled SI-I to SI-8, were observed in the HTCVD and/or PVT 4H-SiC substrates. Photo-EPR has revealed several deep levels responsible for the SI properties in different types of SI 4H-SiC. Annealing behaviour of the defects and the stability of the SI properties with high temperature annealing were also studied.