This paper provides additional experimental data on the neutron ﬂux effect on RPV hardening and embrittlement and on the so-called unstable matrix damage that was suggested to occur at high ﬂux. Six materials taken from the ﬁrst irradiation surveillance capsules of Belgian PWRs with a ﬂuence not exceeding about 1.5 E19 n/cm² were further irradiated in the BR2 high ﬂux reactor to additional ﬂuences ranging between about 1 and 1.5 E20 n/cm² at 290 °C. Eight additional RPV materials were selected to investigate the ﬂux effect on irradiation hardening. No statistically-signiﬁcant difference in irradiation hardening for low and high ﬂux could be evidenced from the null hypothesis test applied with the general linear model. This is conﬁrmed by additional experiments where fourteen irradiated specimens of various RPV materials consisting of low to high Cu and Ni contents were annealed at 350 °C for 5 h to eventually reveal some recovery of the unstable matrix damage. The results did not show any recovery upon heat treatment, which indicates that unstable matrix defects did not appear in these materials during irradiation at high ﬂux.