Two positron annihilation spectroscopy (PAS) techniques have been used for the investigation of helium behavior in STIP samples. Positron lifetime measurements and coincidence Doppler broadening spectroscopy have been employed together in a complex PAS characterization of RAFM steel irradiated in a mixed neutron–proton spectrum up to 20dpa and 1800appm He. Both techniques show an increase of the He-to-dpa ratio up to ∼10dpa. At higher irradiation loads, the ratio is decreasing, which was attributed to the formation and growth of helium bubbles.