Electron spin resonance (ESR) on thermal (100)Si/SiO2 predominantly exhibiting either the Pb0 or Pb1 interface defect confirms the Pb1 point symmetry as monoclinic-I with g1=2.0058, g2=2.00 735±0.00 010, and g3=2.0022, where the g2 direction is at 3°±1° (towards the interface) with a <111> direction at 35.3° with the interface plane. Its line width is found weakly dependent on magnet angle, exhibiting a strain induced spread σg⊥∼0.00 035 in g⊥ about 2–3 times less than typical for Pb in (111)Si/SiO2. For Pb0, an axially symmetric g matrix is observed, with g||=2.0018 and g⊥=2.0081, and σg⊥∼0.0009. From comparison of salient ESR data, it is concluded that Pb and Pb0 are chemically identical; however, systematic fabrication-induced variations in defect environment will lead to second order systematic shifts in average properties. The Pb1 defect is provisionally pictured as an unpaired Si bond on a defect Si atom at slightly subinterface plane position in the Si substrate, possibly facing an oxygen atom. © 1998 American Institute of Physics.