Iron alloys (Fe, Fe-0.7Ni, Fe-0.7Ni-0.025P) were irradiated at 288°C to a fluence of 4.63×1019 neutrons/cm2. Analysis of the as-irradiated alloys showed irradiation-induced defects in the form of interstitial prismatic dislocation loops. The defect size decreased and the defect number density increased with increasing impurity content. The pure iron alloy contained dislocation loops predominantly with an a<100> Burgers vector. The Fe-Ni and Fe-Ni-P alloys, however, contained loops predominantly with an a/2<111> Burgers vector. Annealing at 500°C increased the size and decreased the number density of dislocation loops in all the alloys. The a<100> dislocation loops seemed to grow at the expense of the a/2<111> dislocation loops. The results are explained by a solute trapping mechanism.