Results are presented of an electron spin resonance (ESR) study carried out at cryogenic temperatures (4.2 K≲T≲35 K) at X‐ and K‐band frequencies of spin‐active defect centers created by ion implantation of 1014 As+ cm-2 at 60 or 80 keV into the SiO2 layer of Si‐SiO2 structures (dSiO2=104–130 nm). Observations were made on single‐crystal boron doped (001) Si wafers of resistivities ρ300 K=0.23 and 22 Ω cm, which had been thermally oxidized. In general, three well‐distinguished centers are observed; two signals, which originate from centers residing in the SiO2 layer, showed typical ‘‘glass’’ pattern line shapes. The first is definitely proven to be the E′ center, which constitutes the first clear experimental evidence of the E′ center in ion‐implanted Si/SiO2 structures. The second signal is shown to correspond very nearly with the ‘‘wet’’ oxygen‐related hole center (OHC) with zero crossing of the derivative spectrum dP/dB given by ga=2.0095. A third previously unreported anisotropic center located at the Si/SiO2 interface appeared after annealing, with g=1.9998±0.0001 (for magnetic field B‖ ) and linewidths ΔBptp=0.42±0.02 and 0.49±0.04 mT at 9.0 and 20.9 GHz, respectively. The influence of annealing in dry N2 at 1000 °C and the temperature dependence of the ESR signals are discussed.