We present the results of a systematic investigation of neutron-irradiated and thermally annealed Fe-Cu-Ni-P model alloys using positron annihilation spectroscopy (PAS), including lifetime and Doppler broadening techniques, and Rockwell hardness. These alloys were examined in the as-fabricated state, after irradiation at 270° C to 1 × 1019 n.cm-2, and to 8 × 1019 n.cm-2, and after successive post-irradiation isochronal anneals at temperatures from 200 to 600° C. The results can be qualitatively explained by invoking an irradiation-induced microstructure consisting of a combination of small dislocation-type defects or defect clusters (matrix damage) and dense precipitation of fine scale irradiation-induced precipitates. The matrix damage anneals between 350° C and 450° C. The irradiation-induced precipitates also evolve with annealing, but at higher temperatures. The combined effect of high Cu and high Ni concentrations leads to more extensive irradiation-induced precipitation than in cases where either element is missing, whereas the effect of P is less pronounced. We analyze and compare the results with similar measurements performed on irradiated pressure-vessel steels and with other positron measurements on model alloys, reported in the literature.