The multi-frequency electron spin resonance (ESR) study of the GePb1 Ge dangling bond (DB) interface defect, appearing identically at both SiO2/GexSi1–x interfaces in condensation-grown SiO2/GexSi1–x/SiO2/(1 0 0)Si heterostructures (0.28 ≤ x ≤ 0.93), shows that the ESR signal width is dominated by inhomogeneous broadening due to a strain-induced spread in g. For the x = 0.73 case, this results in a frequency (ν)-dependent peak-to-peak broadening of ΔBppSB/ν = 0.62 G/GHz for the applied magnetic field B along the g3 principal axis Ge DB direction. Compared to the familiar Si Pb-type interface defects in (1 0 0)Si/SiO2, the enhanced ν-dependent broadening scales with the spin–orbit coupling constant ratio λ(Ge)/λ(Si). The natural inhomogeneous broadening due to unresolved 73Ge hyperfine interaction is found to be below ∼1.56 G. While the results adduce quantitative support for the assignment of GePb1 as a Ge DB-type interface center, it is concluded that the GeSi/SiO2 interface uniformity is of good level.