Electron paramagnetic resonance measurements of high-purity semi-insulating 4H SiC reveals a spectrum characteristic of an S=1 defect, which appears only after exposure to light with a wavelength less than 690 nm. Analysis of the hyperfine structure of the spectrum suggests that the defect is an intrinsic pair or defectimpurity complex. The center is stable in an inert ambient up to temperatures of 1200°C, but a 1-h, 1600°C anneal reduces the concentration by at least an order of magnitude. Because the spectrum is not affected by removal of the excitation light, it is concluded that the center is the ground state of an S=1 defect. A study of the angular dependence of the spectrum shows that g=2.0052, ∫D∫ = (329±14) ×10-4 cm-1, and ∫E∫ <19×10-4 cm-1. © 2005 American Institute of Physics.