The hyperfine spectrum associated with unpaired electrons at the (111) Si/SiO2interface (Pbcenters) is reported for the first time. Electron paramagnetic resonance measurements indicate that the hyperfine interactionS&drarr;⋅A¯⋅I&drarr; arises from the29Si isotope and is characterized byA∥=146.(±5.)×10−4cm−1andA⊥=85.(±8.)×10−4cm−1. An analysis of this hyperfine interaction firmly establishes many of the details in the structure of this interface defect.