A survey of neutron- and thermally-induced phosphorus inter-granular segregation behavior in C-Mn welds, LWR plates, LWR HAZ materials, and VVER steels is described in this paper. The materials were irradiated up to doses of 0.13 dpa (fast neutron energy > 1 MeV), with dose rates ranging from 1.75 × 10-8 dpa/s to 10-12 dpa/s, at temperatures between 190°C and 400°C. Irradiation-induced P inter-granular segregation has been found in LWR and VVER materials at high doses (generally 10–130 mdpa) but is absent in all Magnox C-Mn submerged-arc welds at lower doses (1.5–13 mdpa). The dose sensitivity of the irradiation-induced P monolayer coverage depends on the material examined, being highest for VVER plates and weld, lower for the LWR materials, and zero for the C-Mn welds. There are high initial P segregations (arising from pre-heat-treatment) in C-Mn weld and LWR HAZ materials and low initial P segregation in LWR and VVER steels. Comparisons of the experimental data with model predictions are made. The amount of phosphorus segregation is shown via site competition models to be related to free carbon concentration. The model results show a good agreement with experimental measurements.