Defects and impurities in high-purity semi-insulating (HPSI) SiC substrates grown by high temperature chemical vapour deposition (HTCVD) and physical vapour transport (PVT) are studied using electron paramagnetic resonance (EPR) and photoluminescence (PL). The carbon vacancy in the positive charge state (V-C(+)) is observed in all HTCVD, and PVT HPSI substrates. EPR signals of (C-Si-V-C) pairs are often detected in HPSI samples. The T-V2a, which was previously attributed to V-Si(0), is often observed with different concentrations in HTCVD material. The (+/0) donor level of V-C at 1.47 eV above the valence band is suggested to be important for the Slproperties of HPSI 4H-SiC substrates with the activation energies E(a)similar to1.4-1.5 eV. The SI-5 center may be related to the vacancy pair in the negative charge state (V-C-V-Si)(-) and its acceptor level (-1/-2) is in the region similar to1.24-1.51 eV below the conduction band. This center is stable at annealing temperature of 1600 degreesC. After annealing, V-C(+) and V-Si-related signals decrease but can still be observed, whereas the (C-Si-V-C) pairs completely disappear.