Semi-insulating SiC grown by the HTCVD technique are studied by luminescence and absorption measurements and the results are compared to PAS and SIMS results. We have found that metal impurities are present but only in very small concentrations. The semi-insulating properties are instead determined by the intrinsic defects, mostly the silicon vacancy in hydrocarbon rich grown material and the carbon vacancy in the hydrocarbon poor grown material. The hydrocarbon poor material is stable upon annealing both from a vacancy concentration point of view and from a resistivity point of view. The hydrocarbon rich grown material does not stand the annealing at 1600 °C and the resistivity is decreased; from the absorption and PAS measurements we have observed that the decrease in silicon vacancy concentration fits the growth of the vacancy clusters.