Abstract The maximum temperature at which the ordered Pd 8V phase forms in a Pd-15at.%V alloy irradiated by 400 keV protons was determined to lie between 350 and 420 °C. Post-irradiation annealing experiments have established that Pd 8V is most probably a thermodynamically stable equilibrium phase with a critical ordering temperature slightly below 400 °C. Pd 8V is not expected to nucleate and grow under conditions of normal isothermal annealing (because of the low temperature range of its stability) unless diffusion is accelerated, in this case by charged particle irradiation. Interstitial dislocation loops are prevalent in the microstructure of samples irradiated at low temperatures (200 °C), whereas more widely extended stacking fault ribbons are commonly observed at higher irradiation temperatures (420 °C). The segregation of undersized vanadium atoms to these defects causes the preferential formation of Pd 8V in regions between them.