Highlights ► Thin MOS capacitors high fluence 1.8 MeV proton irradiation effects are investigated. ► C– V curves of 7 nm n-type MOS capacitors are measured before and after irradiation. ► A decrease of accumulation regime capacitance is shown at fluences up to 5 × 10 12 cm −2. ► It is the first time that such device effect is observed within the LHC fluence range. ► This effect is mainly due to a proton-induced increase of the substrate resistivity.