An electron paramagnetic resonance spectrum, observed in electron irradiated silicon and labeledSi‐G15, is shown to originate from the same carbon‐oxygen complex as does the well studied C‐line photoluminescence spectrum with zero‐phonon line at 0.79 eV. Both thegtensor and the13C hyperfine tensor for this center are remarkably similar to those for the isolated interstitial carbon atom. Stress‐induced alignment experiments reveal the role of oxygen and indicate a unique structure for an interstitial carbon‐interstitial oxygen pair.